US Secretary of State John Kerry presents the administration's case for US military action against Syria to a Senate Foreign Relations Committee hearing in Washington September 3, 2013. [Photo/Agencies]

ACCELERATING HUMAN COST

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.

Top House Democrat Nancy Pelosi also voiced support for military strikes after meeting Obama, but he will still have to persuade some lawmakers, including Democrats, who have said they are concerned the president's draft resolution could be too open-ended.

US Secretary of State John Kerry and Secretary of Defence Chuck Hagel took the administration's message to a Senate Foreign Relations Committee hearing, where they were pressed on whether the resolution put to Congress would explicitly rule out the use of ground troops.

Kerry said the language of the resolution was still being worked out, but it was important to leave options open for using troops in a scenario where "Syria imploded" and stockpiles of chemical weapons needed to be secured from extremists.

"I don't want to take off the table an option that might or might not be available to the president of the United States to secure our country," he said at the hearing.

When some senators objected to the idea of "boots on the ground", Kerry said the administration would work with Congress to draft a resolution that addressed concerns about ground troops.

"I know the administration has zero intention of putting troops on the ground and within the confines of this authorization, I'm confident we'd have zero problem with including some kind of prohibition there if that makes you comfortable," he told the senators.

A 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.

The UN 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, UN 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.

Russia, backed by China, has used its veto power in the UN Security Council three times to block resolutions condemning Assad's government and threatening it with sanctions. Assad, like Russia, blames the rebels for the August 21 gas attack.