BEIRUT: Syria blamed "terrorist" bomb-makers on Thursday for an explosion that ripped through a building and killed 16 people in the restive city of Hama, where hostility to President Bashar al-Assad runs deep.

The Syrian Observatory for Human Rights, a British-based anti-Assad organization tracking the 13-month-old conflict in which at least 9,000 people have died, gave the same death toll but said the cause of Wednesday afternoon's blast was not clear.

The Local Coordination Committees, a grassroots opposition group, had said earlier that an army rocket had inflicted the carnage and put the death toll at more than 50.

Whatever its origins, the explosion deals another blow to a two-week-old U.N.-backed truce that has failed to halt violence, although somewhat reducing its intensity from previous levels.

United Nations monitors are trickling into Syria to check the ceasefire engineered by U.N.-Arab League envoy Kofi Annan.

Two of them are based permanently in Hama, where many thousands of people were killed when Assad's late father Hafez al-Assad crushed an armed Islamist uprising 30 years ago.

State television showed four corpses from what it said was the Hama explosion, as well as footage of severely wounded children being treated on make-shift hospital beds.

The Syrian government bars most independent media from the country, making it hard to verify events on the ground.

Syrian activists have been dismayed at the pace of the U.N. observer deployment - a senior U.N. official said this week it would take a month to put the first 100 monitors on the ground.

Only 15 are in place so far out of an envisaged full-strength team of 300 to be led by Norwegian General Robert Mood.

The killing of a Syrian Arab Red Crescent volunteer on Tuesday underscores the dangers the monitors may face.

The International Committee of the Red Cross said three other aid workers were wounded when the clearly marked ambulance in which they were travelling came under fire near Damascus. SANA, the state news agency, blamed an "armed terrorist group".

Syria says it has completed withdrawing tanks and troops from populated areas in line with Annan's peace plan, but the former U.N. chief said on Tuesday Damascus had failed to meet all its commitments and the situation remained "unacceptable".

France, leading Western calls for tougher action against Assad, says it plans to push next month for a "Chapter 7" Security Council resolution if Assad's forces did not pull back.

Chapter 7 of the U.N. charter allows the Council to authorize actions which can include the use of military force.

But Western powers have disavowed any intention to intervene militarily in Syria, as they did last year in Libya. AGENCIES