“This has now been going on for nearly two-and-a-half years. We really shouldn’t be in the business of ruling out any options.
“There are no palatable options, I want to be clear with the whole country about that.”
David Cameron faces growing political opposition at home amid suggestions that he is in favour of joining the Americans in helping to assist rebels. He has been warned that he could be defeated in the Commons if he tries to win a parliamentary agreement for Britain to arm the rebels.
Mr Cameron clashed with the Russian president at a Downing Street press conference on Sunday.
Asked by reporters whether he had “blood on his hands” for arming the Assad regime, Mr Putin said that his nation had acted in accordance with international law by delivering arms to the Syrian government.
He added: “I believe you will not deny the fact that one should hardly back those who kill their enemies and eat their organs – all that is filmed. Do you want to support these people? Do you want to supply arms to these people?”
The Deputy Prime Minister Nick Clegg also cautioned Mr Cameron against arming the Free Syrian Army, saying that if it were a good idea, Britain would have done it already. The former head of the Army, Lord Dannatt, said he feared any such assistance would lead Britain into further intervention, while the Archbishop of York, Dr John Sentamu, urged Mr Cameron to “tread very warily”.
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.
“President Assad wants us to think that the only alternative to him is extremism and violence. Yet there are millions of people in Syria who want a peaceful and democratic future. We should be on their side."
Clip courtesy of Today on BBC Radio 4, the full interview is available to listen to on the BBC's iPlayer.