David Cameron and William Hague have held high-level international talks to ratchet up pressure on Syrian President Bashar Assad in the wake of the massacre of more than 100 people, including women and small children.
The foreign secretary flew to Moscow for talks with opposite number Sergei Lavrov to try to secure Russian backing for a fresh United Nations Security Council resolution against the Assad regime and to warn of the danger of "all-out civil war" in the Middle Eastern state.
But continuing differences between Russia and the West surfaced, as Mr Lavrov said it appeared both the regime and rebel forces had a hand in the bloodshed and declined to join Mr Hague in calling for Assad to stand down, the Press Association reported.
Syria's claims that its troops came under attack from heavily-armed rebels and were not to blame for the massacre in the town of Houla have otherwise been given short shrift, including by the UN, which last night accused the Assad regime of "an outrageous use of force".
The regime's charge d'affaires in London was summoned to the Foreign Office for a dressing-down over what political director Sir Geoffrey Adams described as "a sickening and evil crime" which had cost the lives of 108 people, including 49 children and 34 women.
Sir Geoffrey told the Syrian diplomat that Britain wanted all those responsible to be identified and held accountable and warned of "further quick and robust action" from the international community unless all military operations ceased immediately.
Former UN secretary general Kofi Annan, who is acting as the international community's envoy on Syria, today voiced his personal horror at the killings, as he visited Damascus for face-to-face talks with Assad.
Mr Annan called on the regime to halt violence and urged "every individual with a gun" to lay down their arms.
Mr Cameron and French president Francois Hollande discussed the situation by phone, reaffirming their "full support" for Annan's six-point peace plan, which envisages a truce leading to dialogue between the government and opposition aimed at a Syrian-led political transition.
The prime minister and Mr Hollande agreed that the Friends of Syria Group should meet in Paris, and said they wanted to work with Russia to find a resolution to the crisis.
Mr Lavrov voiced Moscow's support for the Annan plan, but made clear that Russia is not seeking Assad's removal, telling a press conference: "For us it's not the most important thing who is in power in Syria. For us the main thing is to provide for the end of all violence."
Russia, which has an important naval base in Syria, has repeatedly shielded Damascus from action by the United Nations Security Council during the year-long uprising which has cost up to 15,000 lives.
Speaking alongside Mr Hague, Mr Lavrov said Moscow wanted Syria to sort out its problems "without foreign interference" and stressed the need to "respect the territorial integrity and sovereignty" of the country.
Foreign Office minister Lord Howell told Parliament there had been a change in the Russian position, but it was unclear whether Moscow would back a fresh UN resolution.
"It's perfectly true that Russia has joined, as all civilised responsible nations must do, in condemnation of these revolting events," Lord Howell told the House of Lords. "But then the question comes beyond that as to whether the UN Security Council is prepared in a united way to take further actions."
The Security Council will discuss Syria again on Tuesday, ahead of a report on Wednesday from Mr Annan.
Downing Street said that the Syrian authorities had treated the ceasefire initiated last month by the former UN chief with "contempt".
A spokesman said: "Faced with this unacceptable situation and the regime's contempt for the ceasefire, the prime minister and (French) president agreed to act together to increase the pressure of the international community on Assad and to bring an end to the bloody suppression of the Syrian people who aspire to freedom and democracy.
"They reaffirmed their full support for the mission of UN and Arab League special joint envoy Kofi Annan, and the effective implementation of its peace plan for Syria.
"Annan must have the means necessary to carry out the mission entrusted to him in very difficult conditions and the regime in Damascus must immediately co-operate fully with him."
Mr Hague acknowledged that Britain and Russia had not always seen eye-to-eye on Syria, but characterised his discussions with Mr Lavrov as "good and frank and honest" and said Moscow had "a role to play" in resolving the situation.
"We are very much agreed that the Annan plan is the best hope for Syria at the moment, the only hope for Syria to try to break the cycle of violence," he said.
The only alternative to the Annan plan was "ever-increasing chaos in Syria and a descent closer and closer to all-out civil war and collapse", warned Mr Hague.
"There needs to be a fundamental change in the approach of the Assad regime if Syria is to be saved from ever greater chaos and disorder, which wouldn't be in the interests of Russia or any other country in the world."
Mr Hollande is expected to press Vladimir Putin to exert his influence on Assad to rein in his security forces when the Russian President visits Paris on June 1.
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