An explosion has destroyed a building in Syria's central city of Hama, reportedly killing up to 69 people, many of them children.
The opposition Local Co-ordination Committees (LCC) said security forces fired a rocket at the building in the Mashaa Attayar area on Wednesday. The Syrian state news agency reported that the explosion was caused by anti-government bomb-makers who had mishandled explosives.
The Syrian Observatory for Human Rights (SOHR), a UK-based rights group, said that it was not sure what had caused the explosion, and it asked UN monitors in the area to investigate.
The group had initially cited witnesses as saying the explosion was caused by government shelling. On Thursday, it reported that 16 people had been killed in the explosion.
Hama, a hotbed of revolt in the year-long uprising against President Bashar al-Assad's rule, has a small team of UN observers, who are preparing the way for a larger UN mission which will monitor the peace plan negotiated by the UN-Arab League envoy, Kofi Annan, two weeks ago.
Al Jazeera's Rula Amin, reporting from Beirut, said the blast poses a direct threat to Annan's peace plan.
"It poses a challenge on how much the observers can actually do. Now you have two observers on the ground but they have yet to tell us what has happened in Hama."
Activists said among the dead were 13 children and 16 women, with more bodies still under the rubble. Footage on YouTube of the blast showed a crowd of men pulling out the bloodied body of a young girl.
Another video shows the collapsed remains of the building, as ash-covered men dig through piles of masonry looking for bodies amid the cries of onlookers.
An activist who asked not to be named said the blast may have been an explosion from inside the building. Speaking to Al Jazeera, Abu Ghazi, an activist and a Hama resident, said the situation was deteriorating.
"Many of the victims are women and children," he said. "People are dying because we don't have the necessary medical equipment or expertise to save those injured."
Abu Ghazi described Mashaa Attayar as a very poor neighbourhood where houses were built on top of each other.
"Many of the houses host refugees from Homs," he said. "I believe this area was targeted because of the daily rallies that took place here."
Asked whether the UN monitors could help to improve the situation for the people in Hama, Abu Ghazi said: "We don't really count on them. They are laughable."
Two days ago, activists said Syrian forces shelled another district of Hama, killing at least 20 people and wounding more than 60.
Syrian authorities say they are committed to international mediator Kofi Annan's April 12 ceasefire agreement, but reserve the right to respond to what they say are continued attacks by "terrorist groups".
Hama is a particularly sensitive site for the opposition. Assad's father, Hafez al-Assad, crushed an armed uprising in Hama 30 years ago, killing many thousands of people and flattening parts of Hama's old city.
Elsewhere, the SOHR said that at least 17 civilians had been killed by government forces on Wednesday. Four security personnel were also killed, reports indicated.
The government, meanwhile, said that a scurity officer had been killed by a roadside bomb in northwestern Syria, and that a similar bomb in Aleppo province had wounded three people, in Wednesday's violence.
Al Jazeera is unable to independently verify reports of deaths in Syria due to strict government restrictions on journalists.
In a separate development on Thursday, the International Committee of the Red Cross confirmed that a volunteer with the Syrian Arab Red Crescent was shot dead and three others were wounded in the town of Douma, on the outskirts of Damascus, on Tuesday.
"We are saddened and extremely shocked by the death of Mohammed al-Khadraa," Dr Abdul Rahman al-Attar, the president of the Syrian Arab Red Crescent, said in a statement released on Thursday. "This is the third fatal incident involving the Red Crescent in less than eight months."
Khadraa was shot and killed on Tuesday in a vehicle clearly marked with the Red Crescent emblem, the statement said. The state news agency reported that Khadraa had been shot by members of an "armed terrorist group".
In-depth coverage of escalating violence across Syria
UN observers have been visiting Douma, where activists say they have come under intense gunfire from government forces, for the last three days.
In another development, the French foreign minister has said the UN Security Council should consider military action in Syria if an international peace plan fails to stop the violence.
Alain Juppe also demanded that 300 UN observers - authorised to go to Syria - be deployed within 15 days. He said Paris would consider peace envoy Kofi Annan's scheduled report on May 5 as a deadline for Damascus to comply with the plan.
"We think this mediation should be given a chance, on the condition that the deployment of the observer mission happens quickly,'' Juppe said after a meeting with Syrian dissidents at his ministry.
Juppe said that Annan's report on May 5 on the state of the UN-backed ceasefire would be "a moment of truth: Either this mediation is working, or it isn't.
"We cannot allow ourselves to be ignored by the regime in place which has adhered to none of the six points of the Kofi Annan [UN peace] plan. We'd have to move into a new phase".
Juppe said France had been discussing with other world powers the prospect of invoking Chapter 7 of the UN Charter, which allows for action that could be militarily enforceable.