In this image taken from video obtained from the Ugarit News, which has been authenticated based on its contents and other AP reporting, smoke and fire fill the the skyline over Damascus, Syria, early Sunday, May 5, 2013 after an Israeli airstrike. Israeli warplanes struck areas in and around the Syrian capital Sunday, setting off a series of explosions as they targeted a shipment of highly accurate, Iranian-made guided missiles believed to be on their way to Lebanon's Hezbollah militant group, officials and activists said. The attack, the second in three days, signaled a sharp escalation of Israel's involvement in Syria's bloody civil war. Syria's state media reported that Israeli missiles struck a military and scientific research center near the Syrian capital and caused casualties. (AP Photo/Ugarit News via AP video)

BEIRUT (AP) — Israel rushed to beef up its rocket defenses on its northern border Sunday to shield against possible retaliation after carrying out two airstrikes in Syria over 48 hours — an unprecedented escalation of Israeli involvement in the Syrian civil war.

Syria and its patron Iran hinted at possible retribution, though the rhetoric in official statements appeared relatively muted.

Despite new concerns about a regional war, Israeli officials signaled they will keep trying to block what they see as an effort by Iran to send sophisticated weapons to Lebanon's Hezbollah militia ahead of a possible collapse of Syrian President Bashar Assad's regime.

Israel has repeatedly threatened to intervene in the Syrian civil war to stop the transfer of what it calls "game-changing" weapons to Hezbollah, a Syrian-backed group that battled Israel to a stalemate during a monthlong war in 2006.

Since carrying out a lone airstrike in January that reportedly destroyed a shipment of anti-aircraft missiles headed to Hezbollah, Israel had largely stayed on the sidelines. That changed over the weekend with a pair of airstrikes, including an attack near a sprawling military complex close to the Syrian capital of Damascus early Sunday that set off a series of powerful explosions.

The Israeli government and military refused to comment. But a senior Israeli official said both airstrikes targeted shipments of Fateh-110 missiles bound for Hezbollah. The Iranian-made guided missiles can fly deep into Israel and deliver powerful half-ton bombs with pinpoint accuracy. The official spoke on condition of anonymity because he was discussing a covert military operation.

Syria's government called the attacks a "flagrant violation of international law" that has made the Middle East "more dangerous." It also claimed the Israeli strikes proved the Jewish state's links to rebel groups trying to overthrow Assad's regime.

Syria's information minister, Omran al-Zoubi, reading a Cabinet statement after an emergency government meeting, said Syria has the right and duty "to defend its people by all available means."

Israeli defense officials believe Assad has little desire to open a new front with Israel when he is preoccupied with the survival of his regime. More than 70,000 people have been killed since the uprising against Assad erupted in March 2011, and Israeli officials believe it is only a matter of time before Assad is toppled.

Still, Israel seemed to be taking the Syrian threats seriously. Israel's military deployed two batteries of its Iron Dome rocket defense system to the north of the country Sunday. It described the move as part of "ongoing situational assessments."

Israel says the Iron Dome shot down hundreds of incoming short-range rockets during eight days of fighting against Hamas militants in the Gaza Strip last November. Hezbollah fired some 4,000 rockets into Israel during the 2006 war, and Israel believes the group now possesses tens of thousands of rockets and missiles.

The Iron Dome deployment followed a surprise Israeli drill last week in which several thousand reservists simulated conflict in the north. In another possible sign of concern, Israel closed the airspace over northern Israel to civilian flights on Sunday and tightened security at embassies overseas, Israeli media reported. Israeli officials would not confirm either measure.

Reflecting fears of ordinary Israelis, the country's postal service, which helps distribute government-issue gas masks, said demand jumped to four times the normal level Sunday.

Israel's deputy defense minister, Danny Danon, would neither confirm nor deny the airstrikes. He said, however, that Israel "is guarding its interests and will continue to do so in the future."

"Israel cannot allow weapons, dangerous weapons, to get into the hands of terror organizations," he told Army Radio.

Israeli defense officials have identified several strategic weapons that they say cannot be allowed to reach Hezbollah. They include Syrian chemical weapons, the Iranian Fateh-110s, long-range Scud missiles, Yakhont missiles capable of attacking naval ships from the coast, and Russian SA-17 anti-aircraft missiles. Israel's airstrike in January destroyed a shipment of SA-17s meant for Hezbollah, according to U.S. officials.

Israeli officials said Sunday they believe that Iran is stepping up its efforts to smuggle weapons through Syria to Hezbollah because of concerns that Assad's days are numbered.

They said the Fateh-110s reached Syria last week. Friday's airstrike struck a site at the Damascus airport where the missiles were being stored, while the second series of airstrikes early Sunday targeted the remnants of the shipment, which had been moved to three nearby locations, the officials said.

None of the Iranian missiles are believed to have reached Lebanon, said the officials, who spoke on condition of anonymity because they were discussing a classified intelligence assessment.

The attacks pose a dilemma for the embattled Assad regime.

If it fails to respond, it looks weak and opens the door to more airstrikes. But any military retaliation against Israel would risk dragging the Jewish state and its powerful army into a broader conflict. With few exceptions, Israel and Syria have not engaged in direct fighting in roughly 40 years.

The airstrikes come as Washington considers how to respond to indications the Syrian regime may have used chemical weapons in its civil war. President Barack Obama has described the use of such weapons as a "red line," and the administration is weighing its options.

The White House declined for a second day to comment directly on Israel's air strikes in Syria, but said Obama believes Israel, as a sovereign nation, has the right to defend itself against threats from Hezbollah.

"The Israelis are justifiably concerned about the threat posed by Hezbollah obtaining advanced weapons systems, including some long-range missiles," said White House spokesman Josh Earnest. He said the U.S. was in "close coordination" with Israel but would not elaborate.

British Foreign Secretary William Hague also seemed to back Israel, telling Sky News that "all countries have to look after their own national security."

Iran condemned the airstrikes, and a senior official hinted at possible retribution from Hezbollah.

Gen. Masoud Jazayeri, assistant to the Iranian chief of staff, told Iran's state-run Arabic-language Al-Alam TV that Tehran "will not allow the enemy (Israel) to harm the security of the region." He added that "the resistance will retaliate to the Israeli aggression against Syria." ''Resistance" is a term used for Hezbollah and the Palestinian Hamas, another anti-Israel militant group supported by Iran.

Iran has provided both financial and military support to Hezbollah for decades and has used Syria as a conduit for both. If Assad were to fall, that pipeline could be cut, dealing a serious blow to Hezbollah's ability to confront Israel.

U.N. Secretary-General Ban Ki-moon spoke to Arab League Secretary-General Nabil ElAraby by telephone Sunday and both shared their "grave concern" over the air strikes, U.N. spokesman Martin Nesirky said.

Ban called on all sides "to exercise maximum calm and restraint, and to act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict," Nesirky said.

Israel appears to be taking a calculated risk that its strikes will not invite retaliation from Syria, Hezbollah or even Iran.

But Salman Shaikh of the Brookings Doha Center in Qatar warned: "All this could lead us into a wider conflict."

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Federman reported from Jerusalem. Associated Press writers Ian Deitch and Diaa Hadid in Jerusalem and Bassem Mroue and Ryan Lucas in Beirut contributed to this story.

WASHINGTON (AP) — The White House asserted Monday that it's highly likely that Syrian President Bashar Assad's regime, not the rebel opposition, was behind any chemical weapons use in Syria. Responding to weekend airstrikes in Syria, the White House also reiterated its view that Israel has the right to protect itself against weapons that could pose a threat to Israelis.

White House spokesman Jay Carney says there is certainly evidence that chemical weapons have been used in Syria. And on Sunday, a member of a U.N. panel investigating events in Syria said there were indications that rebel forces had used the nerve agent sarin.

But Carney questioned that claim.

"We are highly skeptical of suggestions that the opposition could have or did use chemical weapons," he said. "We find it highly likely that any chemical weapon use that has taken place in Syria was done by the Assad regime. And that remains our position."

Carney's comments came as Syria remained one of the most high-profile issues confronting the administration. Air strikes over the weekend on alleged Hezbollah-bound weapons in Syria and the status of chemical weapons use kept the country's civil war at the forefront. Lebanon's Hezbollah militia is an ally of Syria and foe of Israel.

The weekend airstrike on a military complex near the Syrian capital of Damascus killed at least 42 Syrian soldiers, a group of anti-regime activists said Monday, citing information from military hospitals.

The Israeli government has not formally confirmed that it carried out the air strikes Friday and Sunday, and Carney referred specific questions about the strikes to Israel.

"Israel certainly has the right to be concerned about the transfer of sophisticated weapons to Hezbollah," Carney said. "And that has been a concern of Israel's for a long time. The transfer of sophisticated weapons to terrorist organizations like Hezbollah is certainly a concern and a threat to Israel, and they have the right to act in their own sovereign interest on ... in response to those concerns."

Asked whether the U.S. had been forewarned about the strikes, Carney said: "We are in close coordination as a matter of course with the Israelis, and continue to be."

Administration officials have noted that Israel aircraft struck Syria in January.

Carney says the U.S. is still looking for conclusive evidence about chemical weapons use in Syria. He said there is no timeline for the investigation.

Secretary of State John Kerry was leaving for Moscow on Monday to discuss the situation in Syria with Russian President Vladimir Putin.

"We have consistently, in our conversations with the Russians and others, pointed clearly to Assad's behavior as proof that further support for the regime is not in the interest of the Syrian people or in the interest of the countries that have in the past supported Assad," Carney said. "We have been clear in the past about our disappointment with Russia over their opposition to resolutions at the Security Council with regards to this matter. But this is an ongoing conversation."