Updated

U.N. observers on Thursday inspected the site of a deadly explosion that flattened a block of houses in the central Syrian city of Hama a day earlier and killed at least 16 people.

The government and the opposition traded blame for the blasts. Syrian state-run media said rebel bomb-makers accidentally set off the explosives, while 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 the U.N. special envoy Kofi Annan, Ahmad Fawzi, said observers visited the site but there was no immediate word on what they saw. A pair of U.N. observers is stationed in Hama, part of an advance team of 15 that is to be expanded in the coming weeks to up to 300.

As the violence in Syria continues despite U.N.-led efforts to implement a cease-fire, the international community has become increasingly impatient with the Assad regime. On Wednesday, France raised the prospect of military intervention in Syria, saying the U.N. should consider harsher measures if a peace plan by Annan fails.

Amateur videos said to be of Wednesday's incident 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 are searching 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 rebel bomb-makers mishandling explosives set off a blast that killed at least 16 people and severely damaged at least six houses.

The Local Coordination Committees, a network of activists, said the destruction was caused by intense shelling from government tanks. "The area was shelled for a long period," said spokesman Omar Idlibi, denying the blast was triggered accidentally by rebels.

A second 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 cannot be sure those reports are accurate, and called for an investigation by U.N. observers.

French Foreign Minister Alain Juppe said Wednesday that France had discussed invoking Chapter 7 of the U.N. Charter, which can be enforced militarily, with other world powers. U.S. Secretary of State Hillary Clinton said last week the United Nations should move toward such a step to allow for measures like travel and financial sanctions and an arms embargo. She didn't mention military action. For more than a year, the U.S. has opposed the further militarization of the situation.

Any such move, however, would likely be blocked by Russia and China, which have twice used their vetoes as permanent Council members to protect Syria from condemnation and remain opposed to military intervention. Western powers, too, don't appear interested in sending forces to another Middle East nation in turmoil.

Russia's Foreign Ministry spokesman Alexander Lukashevich said Thursday that violations of the cease-fire were still being committed by both sides, but blamed the opposition.

"Most often this occurs because of provocative actions from the armed opposition, which often force the Syrian security forces to open fire in response," he 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.

Bassma Kodmani, a spokeswoman for the opposition Syrian National Council, called Thursday for a unified Arab stand against what she said was Syria's failure to honor terms of Annan's peace plan. She said the Council wanted 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.

Kodmani spoke to reporters in Cairo ahead of a meeting of Arab foreign ministers scheduled for later Thursday in the Egyptian capital.

Kodmani said she told Arab League chief Nabil Elarabi that Arab foreign ministers at the meeting in Cairo must "rise above their differences" and send a strong message to the Syrian regime to immediately halt violence against civilians.

Arab countries are divided on their response to 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.

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Associated Press writers Karin Laub in Beirut and Frank Jordans in Geneva contributed to this report.