The Syrian government and rebels have traded blame for a huge explosion that killed 16 people in the city of Hama.
It comes as a two-week-old UN backed ceasefire looks increasingly fragile.
Syria blamed "terrorist" bomb-makers for yesterday's blast. Information Minister Adnan Mahmoud said Damascus was "reserving the right to respond to any violation or attack".
The Local Coordination Committees, a grassroots opposition group, said more than 50 people had been killed by what it said was a military rocket.
The British-based Syrian Observatory for Human Rights said the cause of the explosion was unclear, but also gave a death toll of 16.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
United Nations Chief Ban Ki-Moon has said that the Syrian government is not complying with the peace plan and has not withdrawn troops and heavy arms.
The blast in Hama has added to doubts about a ceasefire brokered by UN-Arab League envoy Kofi Annan, who has said Mr Assad failed to order his troops and tanks back to barracks as promised.
Western powers have said they intend to push for an arms embargo and sanctions if Damascus fails to comply with the Annan plan.
Russia and China have made clear that they would veto any attempt to authorise Libya-style military action in Syria and have resisted the idea of sanctions.
The Western powers on the Security Council have signalled that there is no appetite in the West for authorising the use of force against Syria.
