"We are going to ask -- before the end of the week -- for a meeting of the Security Council, probably at a ministerial level ... to try and stop the massacres and prepare for the political transition," Fabius told French RTL radio on Monday.

In London, the top diplomat at the Syrian Embassy resigned his post, the British Foreign & Commonwealth Office said. Charge d'Affaires Khaled al-Ayoubi told British officials that he was "no longer willing to represent a regime that has committed such violent and oppressive acts against its own people," the office said in a statement.

In Washington, the White House said President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan "shared their growing concerns about the Syrian regime's ruthless attacks against its own people, most recently in Aleppo, and the deteriorating humanitarian conditions throughout Syria as a result of the regime's atrocities" in a telephone call Monday.

"The two pledged to coordinate efforts to assist the growing numbers of displaced Syrians, not only within Syria, but in Turkey and the broader region. The president acknowledged the generosity of the Turkish people in hosting so many Syrians who have fled their homes in search of safety in Turkey."

And in Tunisia, his first stop on a visit to the Middle East, U.S. Defense Secretary Leon Panetta told CNN that al-Assad "knows he's in trouble, and it's just matter of time before he has to go."

Asked what he'd say to the embattled Syrian leader, Panetta said, "I would say if you want to be able to protect yourself and your family, you better get the hell out now." Sunday, he predicted that the crackdown in Aleppo will prove "a nail in Assad's coffin" by turning even more people against the government.

The United States is providing nonlethal aid to the rebels, including communications gear. Other countries are providing more direct miltary aid, "So there is no question that one way or another, they are getting the support they need in order to continue this fight," Panetta said.