US seeks momentum for new Syria peace push

US Secretary of State John Kerry was trying to build momentum behind a new drive to end the Syrian civil war on Sunday after high-level talks with Russia and the country's neighbours.

Kerry was due to fly to London to brief Washington's European allies after "brainstorming" talks in Lausanne with the main players in Syria's bloody five-year-old conflict.

The Swiss meeting did not produce a concrete plan to restore the truce that collapsed last month amid bitter recriminations between Washington and Moscow and new fighting on the ground.

But Kerry insisted the new, leaner contact group had come up with some plausible ideas that would be fleshed out in the coming days and might lead to a new, stronger ceasefire.

"The way it wrapped up was to have several ideas that need to be quickly followed up," he said after talks with Russia, Iran, Iraq, Qatar, Jordan, Saudi Arabia, Egypt and Turkey.

"The next contact on trying to follow up on this is going to be immediately, because this is urgent, and we're not letting any grass grow under our feet."

But he said it was too early to reveal what the ideas were, and that high-level contacts - but not a ministerial-level meeting - would continue on Monday to develop them.

He was expected, however, to raise the issues with Britain's Foreign Minister Boris Johnson and senior European colleagues, after flying to London later on Sunday.

Britain, France, Germany, and Italy are members of the International Syria Support Group and have met before with other countries interested in resolving the Syrian crisis.

But US officials now say the full group is too unwieldy to make rapid decisions, and that Saturday's Lausanne meeting was more productive for being focused on the main regional players.

The US envoy's tone was upbeat, but diplomats from all sides warned against hopes of a rapid ceasefire.