David Cameron has urged world leaders to "engage" with Syrian opposition groups as he warned the country was on the brink of a full-scale civil war.
The Prime Minister praised interventions by Turkey and the Arab League against Bashar Assad's brutal suppression of protests, believed to have killed more than 3,500 people.
He called on other states to exert "concerted pressure" on the regime.
Speaking alongside Turkish President Abdullah Gul in Downing Street, Mr Cameron said: "Today we had important discussions on Syria, where now a full-scale civil war is a real possibility.
"Turkey and the Arab League have shown welcome leadership and given us a way to stop the brutality of this morally bankrupt regime. The world now needs to get behind with concerted pressure on the regime and positive engagement with the opposition, who can represent Syria in an inclusive transition."
Earlier, Turkey's prime minister Recep Tayyip Erdogan called for Assad to step down.
"For the welfare of your own people and the region, just leave that seat," he said in a televised speech.
"If you want to see someone who has fought until death against his own people, just look at Nazi Germany, just look at Hitler, at Mussolini, at Nicolae Ceausescu in Romania," he said. "If you cannot draw any lessons from these, then look at the Libyan leader who was killed just 32 days ago."
The warning came the day after Syrian soldiers reportedly opened fire on at least two buses carrying Turkish citizens, in apparent retaliation for criticism by Damascus's former ally.
Foreign Secretary William Hague held talks with Syrian opposition groups on Monday, urging them to agree a "common platform" for the country's future.
