By Patricia Zengerle and Khaled Yacoub Oweis

WASHINGTON/AMMAN (Reuters) - U.S. President Barack Obama's effort to win legislative backing for military strikes against Syria passed its first hurdle on Wednesday when a Senate committee voted in favor, but the narrow margin of victory showed the depth of U.S. caution.

In a possible sign of internal unrest in Syrian President Bashar al-Assad's ruling Alawite sect in the shadow of a likely U.S. intervention, Syrian opposition figures said General Ali Habib, a former defense minister, had defected. Syria denied the report.

Washington and Syria's main backer, Russia, remained publicly at odds as Obama tried to build his case for military action over chemical weapons before flying to Russia for a G20 summit hosted by President Vladimir Putin on Thursday.

Putin said U.S. congressional approval without a U.N. Security Council resolution would be an act of aggression, and accused U.S. Secretary of State John Kerry of lying by playing down the role of the militant group al Qaeda with rebel forces.

With Obama focused on building international support, administration officials kept up their campaign of persuasion in Congress, where deep U.S. skepticism about going to war was reflected in a House of Representatives hearing.

Still, after much jockeying over the exact wording, the Senate Foreign Relations Committee approved a resolution authorizing the use of military force in Syria in a vote that avoided party lines, with Democrats and Republicans on both sides. The action cleared the way for a vote in the full Senate, likely next week.

The committee voted 10-7 in favor of a compromise resolution that sets a 60-day limit on any engagement in Syria, with a possible 30-day extension, and bars the use of U.S. troops on the ground for combat operations.

The administration is trying to balance the views of many in Congress who want a narrowly defined resolution against hawks such as Senator John McCain, who has pushed for a broader resolution that would allow direct U.S. support for rebels.

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The Senate committee adopted amendments proposed by McCain with policy goals of degrading Assad's ability to use chemical weapons, increasing support for rebel forces and reversing battlefield momentum to create conditions for Assad's removal.

The authorization still faces significant resistance in Congress, where many lawmakers fear it could lead to a prolonged U.S. military involvement in Syria's civil war and spark an escalation of regional violence.

The full Senate is expected to vote on the resolution next week. The House of Representatives also must approve the measure.

RUSSIAN TENSIONS

Obama said he would continue to try to persuade Putin of the need for punitive strikes on Assad for using chemical weapons when the two meet in St. Petersburg.

In Stockholm en route to Russia, Obama said the credibility of America and of the world was at stake. He appeared to take umbrage at a reporter's question about the "red line" he set for Assad at an August 2012 White House news conference.

"I did not set a red line. The world set a red line," Obama said, referring to bans on chemical weapons use.

Putin again questioned Western evidence. He accused Kerry outright of lying when, in urging Congress to approve strikes on Syria, Kerry played down the role of al Qaeda in the rebel forces. "Al Qaeda units are the main military echelon, and they know this," Putin said.

"He is lying and knows he is lying. It's sad."

Earlier, Putin had said in a pre-summit interview with the Associated Press that he could not absolutely "rule out" Russia supporting a U.N. Security Council resolution to punish Assad - if it could be proved he had used poison gas.

Briefing members of Congress in Washington, Kerry said those comments were "hopeful" and "there may be a road forward where Russia would consider not blocking action."

Kerry played down concerns that any U.S. military strike over chemical weapons might provoke a clash with Russia.

"Foreign Minister (Sergei) Lavrov has made it clear ... Russia does not intend to fight a war over Syria," Kerry told a hearing of the House Foreign Affairs Committee.

A senior Western official said that while Moscow was unlikely to say so in public, there were signs Russian officials believe Assad was responsible for the deaths on August 21 and that it had strained Russian support for him - providing an opening for a new, concerted drive to end the conflict.

However, Putin's characteristically blunt tone towards the U.S. position appeared to limit prospects for a breakthrough in a stalemate that has prevented international action to rein in a conflict that has killed more than 100,000 Syrians and left millions homeless but which neither side has been able to win.

"DEFECTION"

Numerous defections over the past two years by senior commanders, either to the rebel Free Syrian Army or into exile abroad, have not led to a collapse of Assad's defenses.

Habib, the former defense minister, had been under house arrest since he resigned in protest at Assad's crackdown on demonstrators in 2011. He managed to reach the Turkish border late on Tuesday with Western help, Kamal al-Labwani of the Syrian National Coalition told Reuters.

Other sources also said Habib had fled but Syrian state television denied he had left his home. Turkey's foreign minister said he could not confirm the general had defected.

The flight of Habib, if confirmed, would lend credibility to suggestions that parts of the Alawite community may be turning against Assad. Previous high-level defections have generally involved Sunni officers.

"Ali Habib has managed to escape from the grip of the regime and he is now in Turkey, but this does not mean that he has joined the opposition. I was told this by a Western diplomatic official," Labwani said from Paris.

A Gulf source told Reuters that Habib had crossed the Turkish frontier late on Tuesday with two or three other people. He was taken across the border in a convoy of vehicles.

Kerry said he did not know if the report of Habib's defection was correct but "there are currently defections taking place. I think there are something like 60 to 100 in the last day or so, officers and enlisted personnel."

In an interview on Britain's Sky News, Bouthaina Shaaban, a senior adviser to Assad, made no mention of the defection. She said the world should wait for the outcome of a United Nations investigation into the use of chemical weapons and blamed groups linked to al Qaeda for the alleged gas attack last month.

MILITARY PLANS

Following the failure of British Prime Minister David Cameron to win parliamentary backing for air strikes last week, Washington has been struggling to build an international coalition for action in the absence of a U.N. resolution.

Kerry told lawmakers that at least 10 countries had pledged to participate in a U.S. military intervention in Syria, but he did not identify them nor say what roles they might play.

France and Turkey are the most significant military powers lining up behind Obama. The French parliament debated Syria on Wednesday, though President Francois Hollande does not need approval for action.

French Prime Minister Jean-Marc Ayrault told parliament that failure to strike Assad would send a message to the likes of Iran and North Korea that they could defy Western powers with impunity, notably over concerns about their nuclear programs.

Obama has won the backing of key figures in the U.S. Congress, including among his Republican opponents.

But in a sign of the tough road still ahead, Democrats and Republicans took both sides in the Senate committee vote. Two Democrats, Tom Udall and Chris Murphy, joined Republicans Marco Rubio, John Barrasso, James Risch, Ron Johnson and Rand Paul in voting no.

In the Senate, Democratic leader Harry Reid is guardedly confident that a majority of the 100 members will vote yes, but is still unsure if he can get the 60 votes needed to overcome Republican procedural roadblocks, aides said.

In the 435-member House, a senior Republican aide predicted that most of the 50 or so Republicans backed by the anti-big government Tea Party movement will vote no. A number of Democratic liberals are also expected to vote against a resolution, placing the final outcome in doubt.

(Additional reporting by Laila Bassam, Yara Bayoumy and Erika Solomon in Beirut, Paul Taylor and John Irish in Paris, Alexandra Hudson in Berlin, Thomas Grove and Darya Korsunskaya in Moscow and David Alexander, John Whitesides and Susan Cornwell in Washington, Steve Holland and Matt Spetalnick in Stockholm, Michael Holden in London; Writing by Alastair Macdonald and Claudia Parsons; Editing by Giles Elgood and Jim Loney)