Syrian government forces clashed with army defectors in the country's north on Wednesday, causing casualties and further enflaming an area near the Turkish border where rebel fighters have tried to seize territory, activists said.

Syria's persistent bloodshed has tarnished efforts by a U.N. team of observers to salvage a truce that started to unravel almost as soon as it was supposed to begin on April 12.

Human Rights Watch accused President Bashar Assad's regime of war crimes during an offensive ahead of the truce, further throwing into doubt his commitment to a peaceful solution to the conflict.

Despite the violence, the international community still sees the peace plan as the last chance to prevent Syria from falling into civil war — in part because no other country wants to intervene militarily.

Both sides have been blamed for thwarting the truce, with Assad's forces trying to repress demonstrators calling for him to step down and an armed rebellion that has sprung up as peaceful protests have proved ineffective attacking his forces. The U.N. says 9,000 people have died since the uprising began in March 2011.

On Wednesday, activists north of the northern city of Aleppo said intense clashes broke out between troops and army defectors in the village of al-Raai. An activist who identified himself only by his first name Ammar said the clashes began Tuesday night and continued Wednesday. He said there were unconfirmed reports of casualties.

The British-based Syrian Observatory for Human Rights said 15 members of the Syrian security forces, including two officers, were killed when they were ambushed by rebels in al-Raai. It said two army defectors also died in the clashes. But the figure could not be independently confirmed.

Syria's official news agency said a member of the country's security forces was killed and three others wounded in a roadside bomb explosion in the central province of Hama. SANA said the bombing on a road between the towns of Tibet al-Imam and al-Latamneh was carried out by a "terrorist" group — a phrase authorities use for rebels fighting the regime.

In the report, Human Rights Watch detailed violence committed by government forces in northern Syria in a two-week period leading up to the cease-fire, bringing into question whether Assad simply used the time ahead of the cease-fire to strengthen his grip on power instead of preparing for peace.

The New York-based international rights group said troops killed at least 95 civilians and burned or destroyed hundreds of houses as U.N. special envoy Kofi Annan was negotiating with the Syrian government to end the fighting. In a 38-page report, the group documented summary executions, killings of civilians and arbitrary detentions and torture that it says qualify as war crimes.

"While diplomats argued over details of Annan's peace plan, Syrian tanks and helicopters attacked one town in Idlib after another," said Anna Neistat, associate director for programs and emergencies at Human Rights Watch.

"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 cease-fire to cause harm," she said.

The report was based on a field investigation conducted in the towns of Taftanaz, Saraqeb, Sarmeen, Killi and Hazana in Idlib province. Some of the incidents cited appear to confirm widespread reports at the time of an offensive in Idlib in early April that triggered a massive wave of refugees who crossed the border to Turkey with horrific accounts of mass graves, massacres and burned out homes. Activists at one point reported about 100 dead in the villages of Taftanaz and Killi.

HRW said the majority of execution-style killings took place during the attack on Taftanaz. It cited nine separate incidents in which government forces executed a total of 35 civilians in their custody. In other cases, government forces opened fire and killed or injured civilians trying to flee the attacks.

Other groups including the U.N.'s top human rights body have condemned Syria for widespread and systematic violations against civilians.

The U.N.-appointed Independent International Commission of Inquiry on Syria has published two reports during the conflict. Last month, it handed U.N. human rights chief Navi Pillay a secret, sealed list of top Syrian officials who could face investigation for crimes against humanity.

An advance team of U.N. observers is on the ground, in hopes of calming the situation. But while the level of violence has decreased since the April 12 cease-fire, Syrian forces continue attacking rebellious areas instead of withdrawing to their barracks, as required under the deal. Rebel fighters also kept up shooting attacks and roadside ambushes targeting troops.