President Barack Obama urged Congress on Tuesday to approve U.S. strikes on Syria soon as the United Nations said two million Syrians had fled a conflict that posed the greatest threat to world peace since the Vietnam war.

Having startled friends and foes alike in the Middle East by delaying a punitive attack on Syrian President Bashar al-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.

Repeating his confidence of winning votes, expected next week, Obama 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" that might end Syria's civil war.

"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."

Assad denies deploying poison gas that killed hundreds of civilians last month. His enemies were dismayed by Obama's decision on Saturday to seek congressional approval before action that he says is necessary to penalize chemical warfare.

The Syrian opposition, which on Tuesday said a forensic scientist had defected to the rebel side bringing evidence of 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.

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.

REFUGEE CRISIS

After two and a half years of war, nearly one Syrian in three has been driven from home by violence and fear.

The U.N. refugee agency UNHCR 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 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, close to a third of all Syrians 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."

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."

Speaking in Geneva, Guterres noted that a total of six million are 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.

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 Tuesday's Le Figaro, Assad told the Paris newspaper: "Everybody will lose control of the situation when the powder keg blows. There is a risk of a regional war."

The rebels have been struggling to hold ground in recent months, 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.

MISSILE JITTERS

While Obama's wait for Congress to return from its summer recess seems to rule out Western military action this week, Israeli forces training in the Mediterranean with the U.S. navy set nerves on edge in Damascus on Tuesday with a missile test that triggered an alert from Assad's ally Russia.

When Moscow 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 defense 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 defense 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.

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 August 21.

However, Hollande said on Tuesday that there would be no French action if the U.S. Congress fails to back Obama.