US, Britain press Russia on Syria

He also played on Russian pretensions to global leadership, to try to budge Putin, who he will meet at the G8 summit in Northern Ireland next month.

The United States and Britain stepped up pressure on Russia over Syria on Monday, but President Barack Obama warned old suspicions could trample new "common ground" on the crisis.

Obama and British Prime Minister David Cameron sought to build momentum behind a new US-Russia sponsored conference on Syria, now expected to take place in June, as they met at the White House.

"Syria's history is being written in the blood of her people, and it is happening on our watch," Cameron said.

"The world urgently needs to come together to bring the killing to an end."

 Cameron, fresh from a trip to Russia, one of Assad's few remaining backers, said he believed Washington, London and Moscow had found "common ground" on the crisis, which has left tens of thousands dead.

Obama said Russia had an "interest as well as an obligation" to help end the violence, saying he had delivered a similar message to President Vladimir Putin, ahead of Secretary of State John Kerry's Moscow visit last week.

He also played on Russian pretensions to global leadership, to try to budge Putin, who he will meet at the G8 summit in Northern Ireland next month.

"Our basic argument is that as a leader on the world stage, Russia has an interest, as well as an obligation, to try to resolve this issue in a way that can lead to the kind of outcome that we'd all like to see over the long term."

But Obama warned it would be tough to keep Russia on board, saying: "I don't think it's any secret that there remains lingering suspicions between Russia and other members of the G8 or the West."

And Obama said that even with Russia aboard, the situation in Syria may simply be too severe to be solved, noting that Iran, Lebanese militia Hezbollah and Al-Qaeda-allied extremists were all involved.

"Once ... the furies have been unleashed ... it's very hard to put things back together."

Cameron was more optimistic, telling National Public Radio, that Kerry made a "real breakthrough" in getting Russia to agree to a conference, though admitted he and Putin had differences on Syria.

But he said the Russian leader was "keen now to move from the generalities of having a peace conference to talking through the specifics of how we can make (this) work."

Obama has resisted directly arming the Syrian opposition but -- with reports Syria has used chemical weapons, which would cross what the US leader has dubbed a "red line" -- is under increasing pressure to do so.

The president said last week that Washington had a moral and national security obligation to stop the slaughter in Syria, but said he wanted more concrete evidence that chemical weapons had been used.