Secretary of State Hillary Clinton said Assad's legitimacy had "nearly run out" and called for a more united international response to the crackdown in Syria.

Syria's exiled opposition, meeting in Turkey, urged Assad to resign immediately and hand power to the vice president until a council is formed to oversee a transition to democracy.

"The delegates have committed to the demands of the Syrian people to bring down the regime and support the people's revolution for freedom and dignity," said a communique issued by 300 opposition figures after two days of talks in Antalya.

Security forces backed by tanks have laid siege to Rastan, a town of 60,000, since May 29 in an effort to crush protests.

The 13 dead were shot by snipers and troops who imposed a curfew, Ammar Qurabi, head of the Syrian National Organization for Human Rights, and lawyer Razan Zaitouna said.

He said his organization had the names of 1,113 civilians killed since anti-Assad protests erupted on March 18.

Syria blames the unrest on armed groups backed by Islamists and foreign powers. Assad has sent security forces and tanks to several flashpoints, including Deraa, Banias and Tal Kelakh, a border town near Lebanon, and now Rastan.