The United States pleaded with President Bashar al-Assad Tuesday to destroy his chemical arms and scheduled top-level talks with Russia on a face-saving way to head off air strikes on Syria.

President Barack Obama meanwhile met lawmakers ahead of a televised national address, in which he will warn that the threat of US force must be maintained to drive genuine diplomacy to end a showdown over a chemical attack in Damascus.

Political and diplomatic leaders were struggling to keep pace with events, after what officials said was a previously undisclosed US-Russian plan to ease the Syria chemical weapons crisis suddenly burst into life on Monday.

Secretary of State John Kerry, the administration's most strident advocate for military action to punish Assad's regime, dramatically addressed the Syrian leader and his Foreign Minister Walid al-Muallem during an online forum.

He urged the two men to "take advantage of this opportunity as a moment to try to make peace in Syria."

"Help us in the next days working with Russia to work out the formula by which those weapons can be transferred to international control and destroyed," Kerry pleaded during the event, hosted by Google.

Muallem had earlier said that Syria, accused by Washington of killing 1,400 civilians in the August 21 attack, was ready to join the international treaty banning chemical weapons.

In an echo of the great power summits of the Cold War era, Kerry will head to Geneva for talks Thursday with Russian Foreign Minister Sergei Lavrov, who he said had some "interesting observations" on the way forward.

"If we can in fact secure all of the chemical weapons in Syria, through this method, clearly that's by far the most preferable, and would be a very significant achievement," he said.

But wrangling between Russia and Washington and its allies over the terms of any eventual deal reflected the nascent nature of the compromise plan and the long-odds bid to force an international consensus.

Obama started the day by calling British Prime Minister David Cameron and French President Francois Hollande to initiative an effort at the UN to codify the plan to dispose of Syria's chemical arms.

He then ventured up Capitol Hill to meet separately with Senate Democrats and Republicans, to tout his plan for a "limited" military response to the use of chemical weapons by the Assad regime.

A White House official said the president told his hosts he would continue to work with them on an eventual document on authorizing action, even as he pursued the Russian proposal.

The national address, from the East Room of the White House at 9:00 pm (0100 GMT Wednesday), represents Obama's best chance to inject clarity into what has been a chaotic Syria policy in recent days, and to steel diplomacy with the threat of force.

Lawmakers said Obama had asked them for a pause in legislative action to allow diplomacy time to work.

A test vote in the Senate had been set for Wednesday, but no votes on military action are now expected until at least next week.

But glimpses of a possible way out of the Syrian crisis appeared to make some lawmakers even less likely to take a tough vote to okay force.

"He still wants approval for a strike, and I think myself and members of Congress are not ready to provide that approval," Republican Senator John Hoeven told AFP.

Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee and an author of a new draft measure authorizing military action only if UN efforts fail, said the pressure of time had abated.

"I don't think we need to rush out with our hair on fire right now," he said.

"But I think the authorization of military force absolutely needs to remain on the table, otherwise I don't think these negotiations will go anywhere."

Obama aides said Russia's late proposal unveiled Monday had led to some hurried rewriting of the script for the address, as Obama hopes the compromise plan will help him out of a tough and isolated political spot.

The president will argue in the speech, expected to last around 15 minutes, that his specific threat of force -- in defiance of US and global opinion -- had brought the Russian proposal to the table.

He will also assure war-weary Americans that any military action that does take place will be "limited" but effective, and nowhere near the scale of exhausting incursions into Iraq and Afghanistan.