"I think he may be stronger than he was a few months ago, but I'd still describe the situation as a stalemate. And yes, you do have problems with part of the opposition that is extreme, that we should have nothing to do with," Cameron said.
Cameron insisted he was still committed to helping the Syrian opposition but admitted its numbers included "a lot of bad guys".
He said: "[Having extremists in the opposition] is not a reason for just pulling up the drawbridge, putting our head in the sand - to mix my metaphors - and doing nothing. What we should be doing is working with international partners to help the millions of Syrians who want to have a free democratic Syria, who want to see that country have some chance of success."
Asked about arming the opposition, the prime minister said: "We're not intervening by supplying weapons, but I think we can with partners ...... To strengthen those parts of the Syrian opposition that really do represent the Syrian people."
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Cameron also denied reports that his wife Samantha - who was deeply moved by the plight of Syrian refugees when she visited a refugee camp in Lebanon - was dictating government policy.
Over 93,000 people have been killed since the Syria crisis started in March 2011, according to the United Nations, as largely peaceful protests against Assad's rule. It escalated into a civil war after opposition supporters took up arms to fight a government crackdown on dissent.