Media playback is unsupported on your device Media caption David Cameron: 'Syria's history is being written in the blood of her people - and it is happening on our watch'

The US and the UK say they will work to strengthen the moderate opposition in Syria and create a transitional body to replace President Bashar al-Assad.

US President Barack Obama said such a body would be the goal of a meeting in Geneva in "the coming weeks".

The US recently won Russian support for the conference.

British Prime Minister David Cameron, in Washington for talks, said there was an "urgent window of opportunity before the worst fears are realised" in Syria.

Since the uprising against President Bashar al-Assad's rule began more than two years ago, at least 70,000 people are believed to have been killed and more than 1.2 million are living outside Syria as refugees.

The UK-based Syrian Observatory for Human Rights said at the weekend that more than 80,000 people had died, a figure cited by Mr Cameron on Monday.

'Onslaught'

Media playback is unsupported on your device Media caption President Obama: The situation in Syria is "a combustible mix"

Mr Cameron arrived in the US from Russia, where he had discussed the Syrian crisis with President Vladimir Putin.

On Tuesday, in a press conference following talks with Mr Obama, he said he welcomed Russian President Vladimir Putin's agreement to join an effort to achieve a political solution to the Syrian crisis.

"There's no more urgent international task than this. We need to get Syrians to the table to agree a transitional government that can win the consent of all of the Syrian people," he said.

But he added: "There will be no political progress unless the opposition is able to withstand the onslaught and put pressure on Assad so he knows there is no military victory."

Mr Cameron said he had not made a decision to arm the Syrian opposition, but cited the UK's push for further flexibility in the EU's arms embargo on Syria.

He also referred to an earlier pledge to double non-lethal support to the Syrian opposition over the coming year, including armoured vehicles, body armour and generators.

"I do believe that there is more we can do alongside technical advice, assistance, help, in order to shape them, in order to work with them," Mr Cameron said.

Chemical weapons

For months now, the noises from Western capitals have vacillated between the cry "Something must be done!" and the forlorn reply "But what?"

Mr Obama said the US would work to increase pressure on Mr Assad, provide humanitarian aid, support the moderate opposition and prepare Syria for a democratic transition.

"Meanwhile, we'll continue to work to establish the facts around the use of chemical weapons in Syria, and those facts will help guide our next steps," he added.

Both the UK and the US have spoken of growing evidence that the Syrian government have used chemical weapons, including the nerve agent sarin.

Mr Obama had previously warned that such a development was a "red line" for possible intervention,

The BBC's North America editor Mark Mardell says the US and Europe are "slowly inching towards arming the rebels" - a complicated move, because there are many different rebel groups, some of which are hostile to the West.

Mr Putin has been portrayed as one of the main obstacles preventing Western countries taking a stronger line on Syria.

Last week, however, the US claimed a breakthrough when the Russian leader agreed to the international peace conference on Syria, which would involve representatives of both the government and the opposition.

But details of who would attend this conference are vague, and no date has been set.