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. The spokesman for 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 part of a truce plan to end the Syrian crisis. A U.N. official said Thursday 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. But he said differences between politicians and military officials in potential contributing nations have slowed the negotiations for more troops. 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. 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. 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, said the destruction was caused by intense shelling from government tanks, and denied that rebels were responsible for the blast. 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. With the violence in Syria continuing despite U.N.-led efforts to implement the cease-fire, the international community has grown increasingly impatient with the Assad regime. U.S. Secretary of State Hillary Clinton said last week the U.N. should move toward such a step to allow for measures like travel and financial sanctions and an arms embargo. For more than a year, the U.S. has opposed the further militarization of the situation.