ANTALYA, Turkey (Reuters/AFP) - Syrian forces killed at least 13 civilians in the central town of Rastan on Thursday, activists said, in the latest attempt to quell a revolt against the 11-year rule of President Bashar al-Assad. US 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. UN genocide prevention experts warned the Syrian authorities Thursday over "apparently systematic and deliberate attacks" on unarmed civilians. Putting new UN pressure on President Bashar al-Assad's regime over his crackdown, UN secretary general Ban Ki-moon's special advisors on prevention of genocide and responsibility to protect civilian populations said they were "gravely concerned" at the growing death toll from the "violent suppression" of protests. "We are particularly alarmed at the apparently systematic and deliberate attacks by police, military, and other security forces against unarmed civilians taking part in the last two months of protests," said the advisors Francis Deng and Edward Luck. Security forces backed by tanks have laid siege to Rastan, a town of 60,000, since Sunday in an effort to crush protests. The 15 dead were shot by snipers and security forces who imposed a curfew, Ammar Qurabi, head of the Syrian Organisation for Human Rights, and lawyer Razan Zaitouna told Reuters. At least 200 people have been arrested. Syria has barred most international media, making it difficult to verify accounts of the violence. Qurabi said some residents had occasionally used guns. "There have been rare instances of people who have seen their parents, wives or children being killed, (people) taking their personal weapons and trying to resist. But they were smothered by the overwhelming and unjustifiable force being used by the authorities," he told Reuters at the meeting in Turkey. He said his organisation 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 protest flashpoints, including Deraa, Banias and Tal Kelakh, a border town near Lebanon, and now Rastan. Four soldiers shot by "armed terrorist groups" in Rastan on Wednesday were buried on Thursday, the state news agency said. While the crackdown on Rastan intensified, authorities began freeing hundreds of political prisoners after Assad issued a general amnesty, the Syrian Observatory for Human Rights said. French Foreign Minister Alain Juppe said the amnesty had come too late and that Syrian authorities had to have a "much clearer and more ambitious" change of direction.