President Barack Obama warned old suspicions between Russia and the West could hamper new "common ground" on ending the crisis in Syria, even as he and his British counterpart upped pressure on Damascus ally Moscow.
Obama and 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, who just returned from a visit 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 he 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.
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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.
Cameron and Obama met amid indications that Assad's regime may not be hustled swiftly out of power, as his army gained ground in the strategic central province of Homs.
The Syrian Observatory for Human Rights watchdog reported that the military, backed by the Lebanese Shiite group Hezbollah, seized much of the strategically vital Qusayr area, which connects the capital Damascus to the coast.
In southern Daraa, which nurtured the Syrian uprising against Assad, the army secured control of the town of Khirbet Ghazaleh, on the route between Damascus and the Jordanian border.
The Observatory said Monday that it has now documented the deaths of some 82,257 people since the beginning of the conflict in March 2011, including 34,473 civilians.
Reverberations meanwhile mounted from a string of deadly bombings in the Turkish town of Reyhanli, which the Ankara government blamed on Damascus.
Thousands of Turks took to the streets to urge their government to rethink its support for rebels fighting Assad, warning that the decision had provoked reprisals against Turkey, including the bombings, which killed 48 people.
Turkish Prime Minister Recep Tayyip Erdogan is due to meet Obama at the White House on Thursday, with Syria also topping their agenda.
In another sign of accelerating diplomacy, the Kremlin said Israeli Prime Minister Benjamin Netanyahu will hold talks on Tuesday with Putin amid concerns Moscow plans to deliver advanced missiles to the Damascus regime.
UN Secretary General Ban Ki-moon will visit Russia Friday to discuss Syria, Moscow said.
Arrangements for peace talks sponsored by Moscow and Washington, which the State Department said Monday would probably now take place in June, rather than this month as first hoped, remain unclear.
Syrian opposition forces said they will consult with backers Saudi Arabia, Qatar and Turkey before deciding whether to take part in the talks.
State Department spokeswoman Jen Psaki meanwhile would also not rule out that key Syria ally Iran could be invited to join the talks aimed at building on a six-point deal reached in Geneva on June 30 last year.