LONDON - Britain will countenance Syrian President Bashar Assad staying in power for a transitional period but he cannot remain in charge long-term, Prime Minister David Cameron said on Tuesday."What America is saying, which I agree with, is that you need a transition but what's clear about that is at the end of that Assad cannot be the head of Syria," Cameron told CBS Television Network."It wouldn't work because you would not be able to defeat ISIL (Islamic State) if that guy is still running the country," Cameron said. "Assad is one of the recruiting sergeants for ISIL."Russia and the United States have agreed to look for a diplomatic end to the Syrian civil war but clashed over the central question of whether Assad should retain power."So far, the problem has been that Russia and Iran have not been able to contemplate the end state of Syria without Assad," Cameron said, adding that he would work with any country to defeat Islamic State militants."In the end, however far apart we might be, whether it is with the Iranians or the Russians, those two countries have an influence in what happens in Syria and we need to convince them that, you know, a new Syria with a different leader would not necessarily be against their interests, but it would help to get rid of ISIL."