BRITAIN is to double its non-lethal military support to Syria to help the opposition withstand the onslaught from Bashar al Assad's regime and to convince him he cannot win by force.

David Cameron made the announcement at a press conference in the White House during his three-day trip to America.

An extra £10 million for non-lethal military aid such as communications equipment, armoured vehicles and generators is to be made available for Syrian rebels as well as £30m more for humanitarian assistance.

The Prime Minister also suggested Russia, while acknowledging it took a different approach to Syria, might be prepared to participate in a peace process, saying there was a political will to find a solution.

Barack Obama, however, appeared less optimistic, saying: "I'm not promising it is going to be successful. Frankly, sometimes once the furies have been unleashed in a situation like we are seeing in Syria, it's very hard to put things back together.

"There are going to be enormous challenges in getting a credible process going, even if Russia is involved because we still have other countries like Iran and non-state actors like Hezbollah that have been actively involved."

Defending his decision to increase aid to the rebels, Mr Cameron told reporters: "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 but would not be providing lethal weapons yet. "We have not made the decision to arm opposition groups in Syria. We have amended the EU arms embargo in order that we can give technical assistance and technical advice," he said.

"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," he added.

Mr Cameron flew to the US for talks with Mr Obama ahead of next month's G8 summit in Northern Ireland. He said the leaders had agreed to "tackle the scourge of tax evasion" by multinational companies.

Describing himself as "an unashamedly pro-business politician", the PM also spoke of a desire for an EU-US trade deal that could boost the UK's transalantic business by £10bn a year.

Last night, he was due in Boston, scene of the recent marathon terrorist attacks.