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 Assad runs deep.
The Syrian Observatory for Human Rights, a British-based anti-Assad organization tracking the 13-month-old conflict in which the United Nations says 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 a military rocket had inflicted the carnage and put the death toll at more than 50.
Whatever its origins, the blast dealt another blow to a two-week-old U.N.-backed truce that has failed to halt violence on both sides of the conflict, one of a string of uprisings in the Middle East and North Africa against autocratic rule.
An activist said seven civilians and two rebel militiamen were killed in fighting in the eastern city of Deir al-Zor, while a resident of Zamalka on the outskirts of Damascus reported intense gun-battles.
"There have been heavy clashes today, really heavy over the past couple hours," the man said. "I couldn't get close enough to see. There are checkpoints everywhere."
Meanwhile the state news agency, SANA, said a school headmaster was blown up in a booby-trapped car in the northern city of Aleppo, and an "armed terrorist group" had shot dead four members of the same family in Erbin near Damascus.
It also said two members of the security forces were killed in Deir al-Zor.
United Nations monitors charged with checking the ceasefire engineered by U.N.-Arab League envoy Kofi Annan are trickling in to and two are now based permanently in Hama, where many thousands of people were killed when Assad's late father, Hafez Assad, crushed an armed Islamist uprising 30 years ago.
Activists have been dismayed at the pace of the observer deployment, and 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.
Sana said four monitors from Russia, Syria's most powerful ally, were on their way.
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 traveling came under fire near Damascus.
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 planned 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 military force. But Western powers have disavowed any intention to intervene militarily in Syria, as they did last year in Libya.
The U.N. is drawing up a major humanitarian effort for more than a million people affected by the conflict. A report seen by Reuters on Thursday said sewage networks had been damaged and water contaminated, setting the stage for outbreaks of water-borne diseases such as cholera.
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