BEIRUT - Syrian rebels killed 15 members of the security forces in an ambush on Wednesday, a monitoring group said, as a human rights organization accused Damascus of war crimes in last month's run-up to a UN-brokered truce.The Britain-based Syrian Observatory for Human Rights, which has been tracking the 14-month-old uprising against President Bashar Assad, said two rebel militiamen also died in clashes that followed the ambush in the northern province of Aleppo.Damascus says 2,600 personnel have been killed by "armed terrorists". Since a UN-backed ceasefire came into effect on April 12, it has cited rebel assaults as justification of its right to respond to "any violation or attack".The United Nations now has 30 blue-helmeted monitors inside the nation of 23 million people, and this week accused both pro- and anti-Assad forces of violating the three-week-old truce.On Tuesday it said it had credible reports of at least 34 children killed since the ceasefire came into effect.Meanwhile, state media said "terrorist" groups had assassinated two soldiers in southern Deraa province, and the official SANA agency reported the funeral of eight soldiers and policemen killed "in the line of duty".War crimes accusationsThe truce brokered by former UN Secretary-General Kofi Annan has led to a small reduction in the daily carnage, especially in cities were monitors are deployed permanently.However, New York-based Human Rights Watch (HRW) accused government forces of going on a killing spree in opposition areas as officials in Damascus were sitting down with Annan to negotiate the terms of the truce in March and early April.In assaults on the northern province of Idlib, troops killed at least 95 civilians - many of them executed in cold blood - and destroyed hundreds of homes, HRW said in a report that accused Damascus of war crimes."Everywhere we went, we saw burnt and destroyed houses, shops and cars, and heard from people whose relatives were killed. It was as if the Syrian government forces used every minute before the ceasefire to cause harm," senior HRW researcher Anna Neistat said.HRW said researchers observed bullet marks on a wall that formed a row 50 to 60 cm (20 to 24 inches) above the floor, roughly the height of a kneeling person.Damascus has not commented on the report. It accuses foreign-backed armed groups of being behind the violence.Assad appeared to throw an olive branch to thousands of draft-dodging conscripts, with the announcement on Wednesday of an amnesty for people who have refused to join an army accused of widespread brutality.Syria is also gearing up for multi-party elections on May 7 - part of a political reform package agreed to by Assad as a gesture towards those who want an end to his family's four-decade grip on power.Western states do not set much faith in either the ceasefire or reform process.Paris has called for UN sanctions, but the West can do little given the diplomatic cover Syria enjoys at the Security Council from China and Russia.Moscow says the rebels are mainly to blame for the continued violence and issued a statement on Wednesday condemning "terrorists" for "a large-scale campaign to destabilize the situation and disrupt ... Annan's plan".Western states are wary of military intervention along the lines of last year's air campaign that helped topple Libya's Muammar Gaddafi because of the greater diplomatic and military complexities of tackling Syria, as well as the potential spillover effects on a volatile Middle Eastern neighborhoodState media carried no news of the attack, the latest in a series of heavy losses inflicted on pro-Assad forces in the last week by some of the disparate militias fighting for his downfall.Twelve soldiers died in a firefight in the eastern town of Deir al-Zor on Tuesday, the Observatory said, and nine more, including security officials, died in twin suicide bombs in the restive town of Idlib on Monday, according to state media.Most independent media are barred from Syria or have their movements restricted, making it hard to verify such reports.The United Nations says Syrian forces have killed 9,000 people in a violent crackdown on mass protests that started against Assad in March 2011. The initially peaceful demonstrations which have since turned into a bloody guerrilla insurgency.