LOS CABOS, Mexico - Russia and China have not agreed to any plan for the removal of President Bashar Assad from power but do recognize the danger of an all-out civil war in Syria, US President Barack Obama said on Tuesday as Assad's forces bombarded the city of Homs and clashed with rebels.

International efforts to halt the violence are deadlocked because Russia and China, which wield vetoes in the UN Security Council, have blocked tougher action against Assad. They say the solution must come through political dialogue, an approach most of the Syrian opposition rejects.

British Prime Minister David Cameron said Putin had shifted his view of Assad during talks with Obama and other world leaders at the Group of 20 summit in Los Cabos, Mexico, and that discussions were now focused on a transition of power in Syria.

But Putin immediately seemed to contradict that notion, telling reporters at the end of the summit: "We believe that nobody has the right to decide for other nations who should be brought to power, who should be removed from power."

Russia has been the staunchest backer of Assad and his military crackdown against militants and protesters in Syria, including supplying arms to the Syrian government.

Speaking at the summit, Obama said Assad has lost all legitimacy and that it was impossible to conceive of any solution to the violence in Syria that leaves him in power. Obama conceded he had failed to make a breakthrough with the leaders of Russia or China despite intensive talks.

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"I wouldn't suggest that at this point the United States and the rest of the international community are aligned with Russia and China in their positions, but I do think they recognize the grave dangers of all-out civil war," he told reporters.

He said it is important for the world community to work with the United Nations and international mediator Kofi Annan "on what a political transition would look like. ... But I don't think it would be fair to say that the Russians and the Chinese are signed on at this point."

German Chancellor Angela Merkel, French President Francois Hollande, Turkish Prime Minister Tayyip Erdogan and Cameron were present with Obama for the talks with Putin.

"There remain differences over sequencing and the shape of how the transition takes place but it is welcome that President Putin has been explicit that he does not want Assad remaining in charge in Syria," Cameron told reporters.

"What we need next is an agreement on a transitional leadership which can move Syria to a democratic future that protects the rights of all its communities," Cameron added.

Russian Foreign Minister Sergei Lavrov said Cameron's statement that Putin does not want Assad to remain in power "does not correspond to reality."

The chief UN monitor for Syria told the Security Council that his military observers were repeatedly targeted by hostile crowds and gunfire at close range last week before his decision to suspend operations, UN diplomats said.

Ship carrying weapons bound for Syria turns back

Separately, a cargo ship off the British coast carrying weapons bound for Syria has apparently turned back towards Russia, Britain's Foreign Secretary said, calling again for a halt to arms shipments to Assad.

The Curacao-flagged cargo ship Alaed, last seen off the north-west coast of Scotland this week, was believed to carrying Russian weaponry to Syria, according to an insurer that said it had withdrawn coverage for the vessel.

The Pentagon said Russia's military was preparing to send three ships to Syria but noted that Moscow's stated intention was to send supplies and personnel to its naval facility in the Mediterranean port of Tartus.

Western nations and their Sunni Muslim allies in the Gulf and Turkey seek Assad's overthrow but are wary of intervention, while Russia, China and Shi'ite Iran - Assad's strategic ally - have protected Assad from a tough international response.

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