The UK will double the non-lethal military support for the Syrian opposition to help them withstand an "onslaught" from Bashar Assad's regime, David Cameron said.
The Prime Minister, speaking in a joint press conference with US President Barack Obama at the White House, said Assad had to realise there could be no military victory for his forces.
Mr Cameron said: "There will be no political progress unless the opposition is able to withstand the onslaught and put pressure on Assad so he knows there is no military victory."
He said the UK would continue to press for changes to the European Union arms embargo and added: "We will double non-lethal support to the Syrian opposition in the coming year. Armoured vehicles, body armour and power generators are about to be shipped."
Mr Cameron said 80,000 people were dead and five million had been forced from their homes as a result of the "brutal conflict" in Syria.
He said: "Syria's history is being written in the blood of her people and it is happening on our watch.
"The world urgently needs to come together to bring the killing to an end.
"None of us have any interest in seeing more lives lost, in seeing chemical weapons used, or extremist violence spreading even further."
He welcomed Russian president Vladimir Putin's agreement to push for a political solution but "formidable" challenges remained.
"There's no more urgent international task than this. We need to get Syrians to the table to agree a transitional government that can win the consent of all of the Syrian people," he said.
The UK would increase efforts to "support and to shape the moderate opposition".
Mr Cameron also promised more humanitarian assistance for those affected by the violence.
"There is now common ground between the US, UK, Russia and many others that whatever our differences, we have the same aim: a stable, inclusive and peaceful Syria free from the scourge of extremism.
"There's real political will behind this. We now need to get on and do everything we can to make it happen."
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