Britain and Russia on Friday sought to forge a joint approach to the crisis in Syria, as US Secretary of State John Kerry said there was "strong evidence" Damascus had used chemical weapons against rebels.
British Prime Minister David Cameron visited Russian President Vladimir Putin at his Black Sea vacation home to talk strategy, days after top US and Russian diplomats agreed to work together on a solution to the bloody conflict.
But in Warsaw, Russian Foreign Minister Sergei Lavrov confirmed that Moscow was continuing to fulfil contracts by delivering military hardware to Syrian President Bashar al-Assad's regime in defiance of calls for a freeze.
The West and Russia have been repeatedly at odds over the Syria conflict, with the United States and Europe accusing Moscow of seeking to prop up Assad and selling him arms.
In cordial comments amid the tropical shrubbery of the garden at his Black Sea residence, Putin said he and Cameron had discussed possible options and joint measures for finding peace in Syria, after more than two years of war.
However there was no sign of an immediate breakthrough, despite the warm tone that was a contrast to the tensions that have bedevilled relations between London and Moscow for years.
"At the initiative of the prime minister, we spoke about the possible options for a positive development of the situation and about practical steps in this regard," Putin said after the talks, according to a Kremlin transcript.
"We have a joint interest in a swift halt to the violence and the creation of the process for a peaceful solution that keeps Syria's territorial integrity and sovereignty," said Putin.
Cameron said it was "no secret" the two sides had different positions on the conflict but said they shared an ultimate aim of halting the violence, allowing the Syrian people to elect a government and preventing a growth in extremism.
Cameron flew to Putin's palm-dotted residence before the British premier's planned meeting with US President Barack Obama at the White House on Monday.
The meeting also allowed Russia and Britain to coordinate positions ahead of the next Group of Eight summit which Cameron is hosting in Northern Ireland on June 17-18.
The war in Syria has cost an estimated 70,000 lives and displaced millions of people, including hundreds of thousands who have fled to neighbouring countries.
The talks with Putin came amid concerns that Russia may be preparing to hand over to Syria sophisticated surface-to-air missiles which will significantly strengthen its defences and complicate any foreign intervention.
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Kerry warned that any such sale would be "potentially destabilising" for the region.
Lavrov, on a visit to Warsaw on Friday, defiantly refused to rule out delivering weapons to Syria by completing the implementation of existing contracts.
"Russia is not planning to sell -- Russia has sold and signed contracts a long time ago, and is completing supplies of the equipment, which is anti-aircraft systems, according to the already signed contracts," he told reporters.
But German Foreign Minister Guido Westerwelle, speaking at the same news conference, insisted that "international arms deliveries to Syria must end."
Earlier this week, Lavrov and Kerry -- in a rare joint initiative over the conflict -- offered to hold an international conference in search of peace in Syria.
The UN-Arab League envoy on the Syria crisis, Lakhdar Brahimi, hailed the bid, which he said had convinced him to stay on in a job which many see as a hopeless mission.
The proposed peace conference will aim to build on the unimplemented Geneva accord agreed by world powers last June. The deal set out a path toward a transitional government without spelling out what Assad's fate should be.
In Washington, Kerry again raised the spectre of chemical weapons used by Assad's forces.
"This fight is about the terrible choices that the Assad regime has made with its willingness to kill anywhere... to use gas, which we believe there is strong evidence of use of," Kerry said during a Google+ hangout.
Two weeks ago, the US government first said the Syrian regime had likely used chemical weapons against its own people, but Obama has said he needs more proof to determine whether a "red line" had been crossed.
In an interview published Thursday, US Vice President Joe Biden said once the use of chemical weapons has been verified, Obama would likely make a "proportional response in terms of meaningful action," without elaborating.
Turkey's staunchly anti-Assad Prime Minister Recep Tayyip Erdogan told NBC news he believed the Syrian regime has used chemical weapons, crossing Obama's "red line," and called on Washington to "assume more responsibilities."
On the ground in Syria, the army warned citizens to evacuate the town of Qusayr ahead of an attack, a military source said on Friday, as demonstrators denounced "international silence" in the face of several alleged mass killings.
Activists said Qusayr is surrounded by government forces on three sides, and that approximately 25,000 residents are believed to still be in the city. The regime has made recapturing it a key objective.