By Tom Miles and Yara Bayoumy
GENEVA/BEIRUT, Sept 3 (Reuters) - The number of Syrian refugees has passed the two million mark, a United Nations agency said on Tuesday, warning that the world faces its greatest threat to peace since the Vietnam war.
As President Barack Obama wrestled with doubters in Congress ahead of votes next week on possible U.S. strikes on Syria, Israeli forces training with the U.S. navy in the Mediterranean set nerves on edge in Damascus with a missile test that triggered an alert from the Syrian government's ally Russia.
Obama has asked lawmakers to back military action to punish Syrian President Bashar al-Assad for killing hundreds of people with poison gas last month - a charge Assad denied on Monday as he warned Washington and its French allies of retribution.
With many Americans, including legislators from his own Democratic party, fearful of embroiling the United States in a third major war in a Muslim country this century, Obama has insisted he is not seeking "regime change" in Syria.
But that is precisely what Syrian rebels and their backers among Washington's Arab allies want as they struggle to hold their ground, let alone advance. According to one opposition report, government forces took the strategic northwestern town of Ariha on Tuesday, though others said the battle was not over.
Assad's enemies point to the toll that two and a half years of war have taken on Syria's people, of whom 100,000 have been killed and nearly one in three driven from their homes in fear.
The U.N. refugee agency UNHCR said in a statement on Tuesday that 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 - had pushed the total living abroad above two 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, that leaves close to a third of all Syrians living away from home.
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."
Speaking of the acceleration in the crisis, he said: "What is appalling is that the first million fled Syria in two years. The second million fled Syria in six months."
At a news conference in Geneva, Guterres noted that a total of six million were displaced by the war: "At this particular moment, it's the highest number of displaced people anywhere in the world. And if one looks at the peak of the Afghan crisis we have probably very similar numbers of people displaced.
"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.
When Russia raised the alarm on Tuesday morning that its forces had detected the launch of two ballistic "objects" in the Mediterranean, thoughts of a surprise strike on Syria pushed oil prices higher on world markets and must have put the troops operating Syria's Russian-equipped air defence system on alert.
A Syrian security official later told a Lebanese television channel that its early warning radar had picked up no threats.
Clarification came only later when the Israeli Defence Ministry said that its troops had - at the time of the Russian alert - fired a missile that is used as a target for an anti-missile defence system during an exercise with U.S. forces.
The jitters reflected a nervousness both within Syria and further afield since Western leaders pledged retribution for the use of chemical weapons.
Obama's surprise decision on Saturday to refer to Congress for approval next week has, however, delayed any U.S. move.
Britain has dropped out of planning for attacks since its parliament rejected a proposal by Prime Minister David Cameron but France, western Europe's other main military power, is still coordinating possible action with the Pentagon.
President Francois Hollande has resisted opposition calls to submit any decision to wage war to parliament. His government presented lawmakers on Monday with what it said was evidence of Assad's responsibility for a "massive and coordinated" chemical attack on rebel-held suburbs of Damascus on Aug. 21.
Assad warned: "Everybody will lose control of the situation when the powder keg blows. There is a risk of a regional war."
Obama's efforts to persuade the U.S. Congress to back his plan to attack Syria met with scepticism on Monday from lawmakers in his own Democratic Party who expressed concern the United States would be dragged into a new Middle East conflict.
"There is a lot of scepticism," said Representative Jim Moran after taking part in a 70-minute phone briefing for Democratic lawmakers by Obama's top national security aides about the response to a chemical weapons attack that U.S. officials say killed 1,429 people two weeks ago.
Obama appeared to make some headway, however, with two influential Republican senators, John McCain and Lindsey Graham, who came out of a White House meeting convinced that Obama is willing to use air strikes not just to destroy Syrian chemical weapons capability but also to bolster Syrian rebels.
McCain, long an advocate of a more robust U.S. approach to Syria, said failure to get behind strikes against Assad's forces would be "catastrophic."
Obama's abrupt decision to halt plans for a strike against Assad's forces and instead wait for congressional approval has generated a raging debate just as the president prepares to depart on Tuesday on a trip centred on a G20 summit in Russia.
The biggest obstacle he faces is winning the support of members of his own party in the House of Representatives and conservative Republicans who see little need for the United States to get involved in distant civil wars.
Secretary of State John Kerry, who was among the Obama advisers on the call for the Democrats, urged support for giving Obama a resolution to use force, saying Syria had reached a "Munich moment", according to participants. At Munich in 1938, Britain and France cut a deal with Nazi Germany to avert war.
With U.S. warships in place and ready to launch cruise missiles on Obama's order, no decision is likely until days after Congress returns from its summer recess on Monday.
Obama's gamble to seek congressional backing carries many risks, chief among them is that Congress will again thwart him and make him and the United States look weak around the world. (Additional reporting by Erika Solomon in Beirut, Stephanie Nebehay in Geneva, Steve Gutterman and Timothy Heritage in Moscow, Jeffrey Heller and Dan Williams in Jerusalem and Steve Holland and Susan Cornwell in Washington; Writing by Alastair Macdonald; Editing by Giles Elgood)