Syria's bloody uprising has claimed 13 more victims when government forces shelled the central town of Rastan in a continuing offensive that belied offers of an amnesty for regime opponents.

Reports from Rastan and nearby Talbisa, north of Homs, described mosques and shops being hit by shellfire. Al-Jazeera reported 60 dead since Sunday in Rastan alone. Talbisa was said to be under siege by tanks, helicopters and snipers.

Further violence seems likely on Friday as protesters mark a "children's Friday" in memory of Hamza al-Khatib, a 13-year-old boy who was killed in the Deraa area and whose battered and mutilated corpse has become a rallying-point for anger at the regime of President Bashar al-Assad.

Ammar Qurabi, head of the Syrian Organisation for Human Rights, referred to the names of 1,113 civilians killed since the protests erupted on 18 March. International media are banned by a government which talks of facing "armed terrorist gangs" not largely peaceful protests.

Syria's uprising is rapidly becoming one of the bloodiest episodes of the Arab spring – and there is no sign that Assad is likely to be overthrown.

Opposition groups reacted sceptically to an announcement of a conditional amnesty under which hundreds of detainees would be freed. Crucially, it is not clear if the authorities intend to release all the 10,000 or more people they are believed to have detained in the past 10 weeks and the thousands who were in jail before that.

Like other gestures made by Assad since the unprecedented crisis began, this was seen as too little and too late to defuse demands for an end to the regime.

Activists meeting in the Turkish town of Antalya called on the president to step down. "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 delegates at the conclusion of a two-day meeting that brought together opposition groups, activists and independent figures from inside Syria and abroad.

The hope is to raise international awareness of the scale of the repression, but there is still no sign of significant concerted action against Damascus beyond sanctions imposed by the US and EU. Hillary Clinton, the US secretary of state, again left open the possibility that Assad may yet change tack. "The legitimacy that is necessary for anyone to expect change to occur under this current government is, if not gone, nearly run out," she told reporters. "If he's not going to lead the reform, he needs to get out of the way," she said. "Where he goes, that's up to him."