By Phil Stewart

WASHINGTON (Reuters) - The most senior U.S. military officer raised the possibility on Tuesday that American troops might need to take on a larger role in Iraq's ground war against Islamic State militants, but the White House stressed they would not deploy on a combat mission.

General Martin Dempsey, chairman of the U.S. military's Joint Chiefs of Staff, said there was no intention now to place American military advisers on the ground in direct combat. U.S. assistance is taking other forms, including air strikes.

Still, Dempsey outlined scenarios in which he might recommend having U.S. troops do more, potentially accompanying Iraqis during complicated offensives, such as a battle to retake the northern city of Mosul from Islamic State fighters.

"It could very well be part of that particular mission - to provide close combat advising or accompanying for that mission," Dempsey said.

Dempsey acknowledged that Obama's "stated policy is that we will not have U.S. ground forces in direct combat."

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Defense Secretary Chuck Hagel, left, and Army Gen. Martin Dempsey, chairman of the Joint Chiefs of S …

"But he has told me as well to come back to him on a case-by-case basis," he said.

Obama said last week he would lead an alliance to defeat Islamic State militants in Iraq and Syria, plunging the United States into a conflict in which nearly every country in the Middle East has a stake.

But Obama also ruled out a combat mission, saying "we will not get dragged into another ground war in Iraq." How exactly America's role might evolve in the open-ended conflict remains unclear, however.

Responding to Dempsey's comments, the White House said Obama’s military advisers had to plan for many possibilities and that overall policy had not changed - that Obama would not deploy U.S. troops in a combat role in Iraq or Syria.

White House spokesman Josh Earnest told reporters that Dempsey was "referring to a hypothetical scenario in which there might be a future situation where he might make a tactical recommendation to the president as it relates to ground troops."

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An Islamic State militant (L) stands next to residents as they hold pieces of wreckage from a Syrian …

Dempsey's spokesman also issued a statement stressing that the four-star general's exchange in the Senate was not about "employing U.S. ground combat units in Iraq."

Dempsey was testifying before the Senate Armed Services Committee, along with U.S. Defense Secretary Chuck Hagel, as the Obama administration makes its case to Congress for broadening operations against the Sunni militants, which would include U.S. air strikes in Syria for the first time.

NO "SHOCK AND AWE' IN SYRIA

The U.S. military's Central Command is due to brief Obama on its plans on Wednesday. Hagel said those plans envision striking the militant group's safe havens in Syria to knock out infrastructure, logistics and command capabilities.

Dempsey said the strikes would degrade the group's capabilities as broader efforts get under way, including training of more than 5,000 Syrian rebels.

"This won't look like a 'shock and awe' campaign because that's simply not how (the Islamic State militants' group) is organized. But it will be a persistent and sustainable campaign," Dempsey told the Senate Armed Services Committee.

"Shock and awe" was a term popularly used to describe the initial air assault on Baghdad in the U.S. campaign to oust Saddam Hussein in 2003, and refers to use of overwhelming force to undermine an enemy's will to fight.

Congress is expected to approve this week a request from Obama for authorization to arm and train moderate Syrian rebels, one part of his program.

Still, Hagel acknowledged the number of Syrian fighters that could be trained over the course of the year would only put the opposition on a path to roll back Islamic State fighters.

"Five thousand alone is not going to be able to turn the tide. We recognize that," Hagel said.

The Senate hearing was repeatedly interrupted by anti-war protesters, shouting slogans such as, "There is no military solution." One protester was escorted out of the room while holding a sign that read: "More war = More extremism."

Senator Angus King of Maine, expressing concern that the United States would be drawn into interminable fights against extremist groups around the world from Iraq to Syria to Africa, said: "This is geopolitical Wack-a-mole."

(Additional reporting by Missy Ryan and Steve Holland; Editing by Bill Trott, Susan Heavey, Bernadette Baum and Ken Wills)

Martin Dempsey says half of Iraqi forces aren't able to help against IS, and the other half need retooling.
Huge hurdle to a fix

Brussels (AFP) - Russia could go even further in Ukraine with the "frozen conflict" strategy it has tried and tested in other countries, and may even end up annexing around half of Ukrainian territory, analysts say.

Kiev has accused Russian President Vladimir Putin of trying to "take the entire Ukraine", a scenario that several experts can picture, believing that the Kremlin is set on a military solution.

Prime Minister Arseniy Yatsenyuk said Saturday that Putin's aim was not just to annex the separatist regions of Donetsk and Lugansk but to eliminate Ukraine as an independent country.

Western countries accuse Russia of having sent more than 1,000 soldiers to fight in Ukraine alongside pro-Kremlin rebels, a charge which Moscow denies.

Since a ceasefire between Kiev and the rebels on September 5, a swathe of territory on the Russian border, from Lugansk to the Sea of Azov, remains outside Ukrainian government control.

Several analysts said Russia is already carving out a mini-state which would allow it to keep up the pressure on Kiev, as it has done in Georgia, where Moscow backed two separatist republics in the conflict there in 2008, and in Transdniestr, the Moldovan region where the Kremlin has also backed separatists.

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Russian President Vladimir Putin listens to the Audit Chamber chairwoman Tatyana Golikova, foregroun …

NATO chief Anders Fogh Rasmussen said Monday that the "long-term ambition of Russia is to establish a zone of influence in its near neighbourhood" and stop bordering countries from joining alliances such as the EU or NATO.

And some analysts believe Putin -- who has taken to using the tsarist-era term Novorossiya, or New Russia, to describe parts of Ukraine -- has ambitions that go much further still.

"How far Russia will go in southern Ukraine depends mainly on calculations in the Kremlin: How many casualties are acceptable? How much damage to relations with the West can be sustained?" said Ulrich Speck, a visiting scholar at the Carnegie Europe think-tank in Brussels.

"And can territory that has been conquered actually be administered under Russian control, or will it be subject to an endless partisan war?"

Researchers from the Warsaw-based Polish Institute of International Affairs expect Ukraine's pro-Russian rebels to mount a new offensive against government forces this winter and have been war-gaming the most likely scenarios.

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A woman cries in front of her damaged apartment building after shelling in the town of Donetsk, east …

The most probable outcome, they say, is Russian intervention in late October to ensure a land corridor 300 kilometres long and 50 kilometres wide (180 miles long by 30 miles wide) to guarantee energy and food supplies for Russian-annexed Crimea.

A second scenario that is "30 percent likely" is that Russia could proclaim the creation of "Novorossiya" in the southern half of Ukraine, supported by 50,000 to 70,000 Russian soldiers, and linking Moldova's Transdniestr to Crimea, they say.

The Ukranian economy would be devastated by any such move, which would cost the country seven seaports including Odessa as well as two nuclear power stations, with an estimated loss of 27 percent of gross domestic product (GDP).

Around one million people live in the region in question.

The gloomiest scenario involves a complete partition of the country with 100,000 Russian troops controlling the industrial east.

"All of these are realistic. It depends on the timescales," Giles Keir of the Chatham House think-tank in London told AFP.

The Royal United Services Institute, a British military think-tank, has long warned of similar scenarios, saying that Russia's huge arms industry depends on Ukraine for 30 percent of its essential components, particularly for aircraft and missiles, which might tempt Moscow to intervene.

Keir said Putin held the advantage because of his "strategic patience" -- largely because Western democracies must worry more about domestic political cycles.

"Russia does things while people are looking away," for example the invasion of Georgia during the 2008 Beijing Olympics, he said.

"The initiative is entirely in their hands."