* Senate committee draft bars deployment of ground troops
* Obama urges swift approval by Congress for Syria action (Adds agreement on draft authorization)
By Jeff Mason and Yara Bayoumy
WASHINGTON/BEIRUT, Sept 4 (Reuters) - President Barack Obama won the backing of key figures in the U.S. Congress, including Republicans, in his call for limited U.S. strikes on Syria to punish President Bashar al-Assad for his suspected use of chemical weapons against civilians.
Leaders of the U.S. Senate Foreign Relations Committee said they reached an agreement on a draft authorization for the use of military force in Syria, paving the way for a vote by the committee on Wednesday. However, the draft is much narrower than the request made by Obama and includes a provision barring the use of U.S. troops on the ground.
Speaking after the United Nations said two million Syrians had fled a conflict that posed the greatest threat to world peace since the Vietnam war, Obama said on Tuesday the United States also has a broader plan to help rebels defeat Assad's forces.
"What we are envisioning is something limited. It is something proportional. It will degrade Assad's capabilities," Obama said. "At the same time we have a broader strategy that will allow us to upgrade the capabilities of the opposition."
Having startled friends and foes alike by delaying a punitive attack on Assad until Congress reconvenes and agrees, Obama met congressional leaders at the White House to urge a prompt decision and assure them it did not mean another long war like Iraq or Afghanistan.
John Boehner, the Republican speaker of the U.S. House of Representatives, and House Majority Leader Eric Cantor both pledged their support for military action after the meeting.
Votes are expected to be held in the Senate and House next week, with the Republican-led House presenting the tougher challenge for Obama.
The House leadership has indicated the votes will be "conscience votes," meaning they will not seek to influence members' votes on party lines. All the same, it would have been a blow to Obama if he had not secured the backing of the top two Republicans.
"I believe that my colleagues should support this call for action," Boehner told reporters.
The president said strikes aimed at punishing the use of chemical weapons would hurt Assad's forces while other U.S. action would bolster his opponents - though the White House has insisted it is not seeking "regime change."
Among other provisions, the Senate Foreign Relations Committee draft, which was obtained by Reuters, sets a 60-day limit on U.S. military action in Syria, with a possibility of a single 30-day extension subject to conditions.
The compromise deal reached by Senator Robert Menendez, the Democratic chairman of the panel, and Senator Bob Corker, the top Republican, includes a provision banning any use of U.S. armed forces on the ground in Syria, according to the draft document.
It requires Obama to consult with Congress and submit to the Senate and House of Representatives foreign relations panel a strategy for negotiating a political settlement to the Syria conflict, including a review of all forms of assistance to the rebels fighting to oust Assad.
Secretary of State John Kerry initially told the committee he would prefer not to bar the use of ground troops to preserve options if Syria "imploded" or there was a threat of chemical weapons being obtained by extremists.
But when Corker, the Republican senator, told Kerry he "didn't find that a very appropriate response regarding boots on the ground," Kerry quickly, and repeatedly, backtracked.
Kerry said he was simply "thinking out loud" and raising a hypothetical situation, but he did not want to leave the door open to sending ground troops to Syria.
"Let's shut the door now," Kerry said. "The answer is, whatever prohibition clarifies it to Congress or the American people, there will not be American boots on the ground with respect to the civil war."
An Reuters/Ipsos poll showed on Tuesday that Obama has failed so far to convince most Americans. Some 56 percent of those surveyed said the United States should not intervene in Syria, while only 19 percent supported action, essentially unchanged from last week.
In remarks that appeared to question the legality of U.S. plans to strike Syria without U.N. backing, Secretary-General Ban Ki-moon said the use of force is only legal when it is in self-defence or with Security Council authorization.
If U.N. inspectors confirm the use of chemical weapons in Syria, the Security Council, which has been deadlocked on the 2-1/2-year Syrian civil war, should overcome its differences and take action, Ban said.
Assad denies deploying poison gas that killed hundreds of civilians last month.
The Syrian opposition, which said a forensic scientist had defected to the rebel side bringing evidence of the Assad forces' use of sarin gas in March, has appealed to Western allies to send them weapons and use their air power to end a war that has killed more than 100,000 and made millions homeless.
COMFORTABLE GOING FORWARD
Obama has said he is "comfortable going forward without the approval of a United Nations Security Council that so far has been completely paralysed and unwilling to hold Assad accountable."
The presence in rebel ranks of Islamist militants, some of them close to al Qaeda, has made Western leaders wary, while at the same time the undoubted - and apparently accelerating - human cost of the conflict has brought pressure to intervene.
Russia, backed by China, has used its veto power in the U.N. Security Council three times to block resolutions condemning Assad's government and threatening it with sanctions. Assad, like Russia, blames the rebels for the Aug. 21 gas attack.
Obama was due to leave Washington on Tuesday for a G20 meeting in Russia. France said foreign ministers of some of the G20 member states will convene on the sidelines of the meeting to discuss Syria.
The conflict has divided the Middle East on sectarian lines, with Shi'ite Iran backing Assad and Washington's Sunni Arab Gulf allies supporting the mainly Sunni rebels. It has also revived Cold War-style tensions between the Western powers and Moscow.
In an interview in Le Figaro, Assad told the French newspaper: "Everybody will lose control of the situation when the powder keg blows. There is a risk of a regional war."
The U.N. High Commission for Refugees said there had been a near tenfold increase over the past 12 months in the rate of refugees crossing Syria's borders into Turkey, Iraq, Jordan and Lebanon - to a daily average of nearly 5,000 men, women and children.
This has pushed the total number of Syrians living abroad to more than 2 million.
That represents some 10 percent of Syria's population, the UNHCR said. With a further 4.25 million estimated to have been displaced but still resident inside the country, close to one third of all Syrians are living away from their original homes.
Comparing the figures to the peak of Afghanistan's refugee crisis two decades ago, U.N. High Commissioner for Refugees Antonio Guterres, said: "Syria has become the great tragedy of this century - a disgraceful humanitarian calamity with suffering and displacement unparalleled in recent history."
"The risks for global peace and security that the present Syria crisis represents, I'm sure, are not smaller than what we have witnessed in any other crisis that we have had since the Vietnam war," said Guterres, a former Portuguese prime minister. (Additional reporting by Tom Miles and Stephanie Nebehay in Geneva, Steve Gutterman and Timothy Heritage in Moscow, Jeffrey Heller and Dan Williams in Jerusalem, Dasha Afanasieva in Istanbul and Phil Stewart, Arshad Mohammed, Susan Cornwell and Andy Sullivan in Washington.; Writing by Claudia Parsons.; Editing by Christopher Wilson, Jim Loney and Raju Gopalakrishnan)