BEIRUT (AP) -- U.N. observers on Thursday inspected the site of an explosion that flattened a block of houses in the central Syrian city of Hama and killed at least 16 people, while the government and the opposition traded blame over the cause of the blast.
Syrian state-run media said rebel bomb-makers accidentally set off the explosives. Anti-regime activists said intense shelling by government forces caused the extensive damage. It was impossible to independently verify the conflicting accounts because President Bashar Assad's regime, facing a 13-month-old uprising, has restricted access for journalists and other outside witnesses.
The spokesman for U.N. special envoy Kofi Annan, Ahmad Fawzi, said observers visited the site but he had no immediate word on what they saw.
Two U.N. observers are stationed in Hama, part of an advance team of 15 monitors who are visiting hot spots to try to salvage a cease-fire that is part of a peace plan aimed at ending the violence and bringing the two sides to the negotiating table. The observer team is to be expanded in the coming weeks to up to 300.
Amateur videos said to be of Wednesday's blasts in Hama showed a large cloud of white and yellow smoke rising from a neighborhood surrounded by green fields. In a later video, dozens of people searched through the debris, including huge chunks of cement and broken cinderblocks. Another clip shows the bloodied body of a little girl being carried through a crowd of wailing men.
The state-run Syrian news agency SANA said rebels mishandling explosives triggered a blast that killed at least 16 people and severely damaged at least six houses.
The Local Coordination Committees, a network of activists, denied that and said it was intense shelling from government tanks that caused the damage. The group put the death toll as high as 70, but that estimate was not confirmed by others.
Another opposition group, the Britain-based Syrian Observatory for Human Rights, said the cause of the destruction was not immediately clear. The Observatory initially cited reports by local residents that they had come under attack from regime forces.
However, the head of the group, Rami Abdul-Rahman, said he could not confirm those reports and called for an investigation by U.N. observers. He said at least 16 people were killed.
With the violence in Syria continuing despite U.N.-led efforts to implement the truce, the international community has grown increasingly impatient with the Assad regime.
U.N. Secretary-General Ban Ki-moon demanded Thursday that the Syrian government immediately comply with its commitment to withdraw troops and heavy weapons from cities and towns, said U.N. deputy spokesman Eduardo del Buey.
The U.S. ambassador to the United Nations, Susan Rice, condemned the Syrian government's continuing intense use of heavy weapons in Hama and elsewhere.
Rice also said Wednesday's Hama explosion appeared to be "the result of intense shelling" though she couldn't say this with certainty.
Russia, one of the regime's main allies, said violations of the cease-fire were still being committed by both sides, but blamed the opposition overall.
"Most often this occurs because of provocative actions from the armed opposition, which often force the Syrian security forces to open fire in response," Russia's Foreign Ministry spokesman Alexander Lukashevich said. Still, he added, the level of violence in the country has declined considerably since the observers arrived.
"All of this allows us to claim that the situation in Syria is starting to improve slightly, although this is a very fragile trend," he added.
The United Nations has so far negotiated for countries to provide 100 unarmed truce monitors to be on the ground in Syria, in addition to civilian support staff, within 30 days of the April 12 cease-fire, a U.N. official said Thursday. But he said differences between politicians and military officials in potential contributing nations have slowed the negotiations for more troops. He did not elaborate.
The official said the U.N. will announce Friday that Norwegian Maj. Gen. Robert Mood is being appointed to lead the team of U.N. observers. He spoke on condition of anonymity because the announcement has not yet been made.
Bassma Kodmani, a spokeswoman for the opposition Syrian National Council, called Thursday for a unified Arab stand against what she said was Damascus' failure to honor terms of Annan's peace plan. She urged the Arab League to "open the door" to a U.N. Security Council resolution that would create safe havens in Syria and allow international relief agencies to operate there freely.
The Arab League later said it would ask the U.N. Security Council to "review" its policy on Syria if the regime there fails to fully and immediately honor its commitment to a cease-fire.
A statement by Arab foreign ministers meeting Thursday in Cairo said the Damascus regime was negotiating while simultaneously "killing its own people." It said Morocco, currently a member of the 15-nation U.N. Security Council, to convey the League's request when the world body meets to discuss Syria on May 5.
Arab countries are divided over how to deal with the Syrian crisis, with Gulf countries led by Qatar and Saudi Arabia in favor of arming the opposition and others like Egypt, Iraq and Sudan preferring a diplomatic solution.
For now, the international community remains united in support of Annan's plan, which calls for a cease-fire, to be followed by talks between the regime and the opposition on a political solution to the conflict that the U.N. says has killed more than 9,000 people.
That plan, however, has been troubled from the start. Syria has failed to enact key parts of the plan, like withdrawing its forces from cities, and its troops have attacked opposition areas, killing scores of civilians since the truce was to begin on April 12. Rebel fighters, too, have attacked military checkpoints and convoys.
Syria's information minister said Thursday that armed terrorist groups have stepped up their attacks since Annan's peace plan went into effect, adding that they have breached the cease-fire plan more than 1,300 times. Adnan Mahmoud said Annan has been informed of those violations.
Associated Press writers Karin Laub in Beirut, Edith M. Lederer at the United Nations and Frank Jordans in Geneva contributed to this report.