Britain and France will push their plans to scrap a Syrian arms embargo at talks in Dublin next week amid fears that the move could fuel the conflict.
David Cameron and French president Francois Hollande, made a joint pitch at an EU summit in Brussels, insisting the Opposition could need "lethal" military support currently outlawed by the EU's own sanctions against the regime of dictator Bashar Assad.
But German chancellor Angela Merkel - and EU foreign policy chief Cathy Ashton - raised concerns trying to tip the military odds more in favour of the rebels could backfire.
Emerging from a Brussels summit originally called to discuss the economic crisis, Mr Cameron said: "It is worth reflecting that we are two years into this dreadful conflict. There are probably over 70,000 people who have lost their lives.
"We have to be frank that what the international community has done so far, of course, has helped in terms of humanitarian assistance... but it hasn't overall worked in terms of stopping this conflict and achieving transition in Syria."
EU foreign ministers recently renewed the current arms embargo until the end of May, extending it to allow "non-lethal" assistance which the UK can offer the Syrian opposition, to include equipment such as armoured vehicles. But London and Paris are pressing to end the embargo altogether - but this requires a unanimous vote of member states and Germany leads the majority which believes a political solution is the only answer.
Mr Cameron said: "Of course people want a political solution. Of course I want a political solution. But this is not an either/or situation. I think we are more likely to see political progress if people can see that the Syrian opposition - which we have now recognised, that we are working with - is a credible and strengthening and growing force."
He rejected arguments that arms should not be sent to Syria for fear that they will fall into the hands of extremist groups opposed to the interests of the West. And he indicated that Britain and France could act unilaterally if they were unable to achieve agreement in the EU.
"Britain is a sovereign country, we have our own foreign, security and defence policies. If we want to take individual action, we think that is in our national interest, of course we are free to do so," he said.
"France and Britain on these issues, we do work well together, we do have a common analysis of what is wrong in Syria. We want to work together with allies in the Gulf to try and help bring about the change that we want to see."