Up to 70 people have been killed in an attack on a house in Hama, according to Syrian activists.
They said several houses in the Masha at-Tayyar district in southern Hama were destroyed by a big explosion.
State media said 16 people died in the blast in a house used as a bomb factory by "armed terrorist groups".
The violence comes despite a UN-brokered ceasefire - part of a peace plan proposed by the joint UN-Arab League envoy Kofi Annan.
Following the blast in Hama, activists posted video on the internet showing a scene of devastation, with bodies being pulled from the rubble.
One report said 13 children and 15 women were among the dead.
They said the blast was caused by government shelling or even a Scud missile attack.
The opposition Syrian National Council has called for an emergency UN Security Council meeting "so that it can issue a resolution to protect civilians".
It says nearly 100 people have been killed in Hama in recent days.
The level of devastation seen would have been difficult to achieve by conventional shelling, the BBC's Jim Muir in Beirut says.
State television showed pictures of injured children in hospital and says that a group using the house to make bombs detonated them accidentally.
The reports cannot be independently verified owing to government restrictions on foreign media.
Meanwhile, a video has emerged which purportedly shows a man being buried alive by security forces, allegedly for sending material to TV stations.
Its authenticity could not be confirmed.
The unnamed man, who is said to be a media activist, is seen pleading for his life as earth is shovelled over his head. He then goes silent.
What appear to be members of the security forces are then heard cursing him for receiving money for sending material to Arabic satellite TV stations.
The video was said to have been leaked by sympathisers.
Continuing violence has been reported across Syria since a ceasefire was introduced earlier this month - including in towns where UN observers are present.
France now says the Security Council should consider the use of force in Syria if Mr Annan's peace plan fails to stop the violence.
The plan calls on Damascus to withdraw troops and heavy weapons from cities.
Mr Annan told the Security Council on Tuesday that the Syrian military had not withdrawn from population centres.
He condemned as "totally unacceptable and reprehensible", reports that troops entered Hama after UN observers departed on Monday, and carried out summary executions as punishment for having spoken to them.
Two observers have now returned to Hama. They form part of a small advance team, ahead of a team of 300 that the UN would like to deploy.
A spokesman for Foreign Minister Walid al-Moualem said, via Twitter, that he had sent a message to Kofi Annan to "set the record straight on several aspects of the Annan plan".
Mr Moualem is reported to have told him that there had been "more than 1,149 documented and verifiable violations from armed elements" since the ceasefire began.
The US permanent representative to the UN, Susan Rice, told reporters on Tuesday that all Security Council members wanted the observers to be deployed quickly.
Ms Rice said that it was hoped 100 observers would be in Syria within a month, but said Syria had made clear it would not admit UN staff from any country in the "Friends of Democratic Syria" group.
The UN says about 9,000 people have died since pro-democracy protests began in March 2011. In February, the Syrian government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.