More than 50 people have been killed in Rastan since a military operation there started at the weekend, reports say.

The offensive comes despite a government amnesty offer and the release of hundreds of detainees.

Opposition groups dismissed the moves and urged President Bashar al-Assad to resign and make way for democracy.

The groups, which comprise about 300 activists, are meeting in Antalya in neighbouring Turkey. They said in a communiqué that Mr Assad should hand power to one of his two vice-presidents, without specifying which, and hold free elections within a year.

The Local Co-ordinating Committee, which helps to organise and document the country's protests, gave the names of the people it said were killed in Rastan in the latest artillery and tank bombardments.

The committee said the offensive had hit at least two mosques and a bakery, as well as houses that collapsed, killing entire families.

Following the announcement on May 31 of a conditional amnesty, hundreds of detainees have been released.

More than 1,000 people have been killed in Syria since an uprising against President Assad began in March, activists say.