David Cameron said he is "as worried as anyone" about terrorist and extremist elements among opposition forces fighting to oust Bashar Assad in Syria.
The Prime Minister was speaking ahead of the opening of the G8 summit in Northern Ireland, which looks set to be dominated by international tensions over Syria.
Among attendees at the summit are US President Barack Obama, who last week said he was ready to send weapons to the Syrian opposition, and Russian President Vladimir Putin, who is the Assad regime's most important international backer and has warned that the West should be wary of arming rebels who "eat the organs" of their enemies.
Mr Cameron insisted that no decision has yet been made on whether Britain should send weapons, and repeated his assurance that he would "never stand in the way" of the House of Commons voting on any such move.
London Mayor Boris Johnson has warned against arming the rebels, saying British weapons could end up in the hands of "al Qaida-affiliated thugs".
In a round of TV interviews at Lough Erne, Mr Cameron said: "Let's be clear - I am as worried as anybody else about elements of the Syrian opposition, who are extremists, who support terrorism and who are a great danger to our world.
"The question is what do we do about it? My argument is that we shouldn't accept that the only alternative to Assad is terrorism and violence. We should be on the side of Syrians who want a democratic and peaceful future for their country and one without the man who is currently using chemical weapons against them.
"What we can try and do here at the G8 is have further pressure for the peace conference and the transition that is needed to bring this conflict to an end."
The Prime Minister added: "We haven't made a decision to give any arms to the Syrian opposition but what we do need to do is bring about this peace conference and this transition, so that people in Syria can have a government that represents them, rather than a government that's trying to butcher them.
"What we are doing right now is helping the official Syrian opposition - people who have signed up to democracy and human rights, who want that sort of future for Syria. We are advising them, helping them and we are assisting them - and we should."