GENEVA (AP) — Syrian President Bashar Assad’s forces and pro-government shabiha fighters have perpetrated war crimes and crimes against humanity on Syrian civilians, a U.N. expert panel concluded Wednesday in a report that provides in chilling detail further evidence of a conflict spiraling out of control.

The panel appointed by the U.N.’s 47-nation Human Rights Council blamed the government and allied militia for the killing of more than 100 civilians in the village of Houla in May, nearly half of them children, and said the murders, unlawful killing, torture, sexual violence and indiscriminate attacks “indicate the involvement at the highest levels of the armed and security forces and the government.”

The panel also concluded in its final report Wednesday to the Geneva-based council that anti-government armed groups committed war crimes, including murder, extrajudicial killings and torture, but at a lesser frequency and scale.

The expert panel appointed to probe abuses in Syria has had hardly any access to Syria, with only its chairman allowed into Damascus. Most of the report, which covers the period between Feb. 15 and July 20, was conducted during field interviews and in Geneva with Syrian refugees outside the country.

The panel conducted 1,062 interviews, but emphasized their lack of ability to carry out their U.N. mandate within Syria hampered their investigation.

The commission is headed by Brazilian diplomat and professor Paulo Sergio Pinheiro and also includes Karen Koning AbuZayd, a U.S. citizen and former head of UNRWA, the U.N. agency that aids Palestinian refugees. A third panel member had dropped out.

Their report, whose findings are more conclusive about the Houla massacre than previous interim findings, could be used by world powers to justify tougher outside action against Syria, or strengthen calls for an international investigation and prosecution of possible war crimes and crimes against humanity.

The council could renew the mandate of the panel or it could appoint Pinheiro to become a special investigator of Syria, a position that the council created in March but has left unfilled until now.

Earlier this year, the council said in a resolution that it agreed with U.N. High Commissioner for Human Rights Navi Pillay in her call for action by the International Criminal Court based at The Hague.

Activists say more than 20,000 people have been killed since the start of Syria’s revolt, inspired by other Arab Spring uprisings against autocratic regimes in the region.

The conflict has slowly changed into a full blown civil war that the panel says involves “more brutal tactics and new military capabilities on both sides.”

Activists on Wednesday reported shelling and clashes in the northern city of Aleppo, Syria’s largest, where rebels took over several neighborhoods over the past weeks. The Britain-based Syrian Observatory for Human Rights said the rebels were trying to take over a key dam in the northern town of Manbij, just east of Aleppo. It added that the army was using helicopter gunships in the battles near the dam on the strategic Euphrates River.

In the northern town of Azaz at least two large explosions leveled dozens of buildings. Associated Press journalists saw at least seven bodies pulled from the rubble. Activists drove some of the wounded to the nearby Turkish border for treatment.

U.S.-based Human Rights Watch said Syrian government fighter planes fired rockets that struck the main emergency hospital in an opposition-controlled area of Aleppo a day earlier, wounding two civilians and causing significant damage. Human Rights Watch said its members visited the damaged hospital.

The Local Coordination Committees, another activist group said there was also fighting near a border crossing with Turkey that the rebels had captured last month. A local official in the Turkish border town of Reyhanli said clashes could be heard coming from the region on Tuesday but that the situation had calmed by Wednesday morning. The official spoke on condition of anonymity because he was not authorized to talk to the media.

Turkey’s state-run Anadolu news agency said 757 Syrians fled their country and streamed into Turkey on Wednesday.

The LCC reported violence in the eastern province of Deir el-Zour, northwestern region of Idlib, Daraa to the south and in suburbs of the capital Damascus.

In Damascus, a bomb attached to a fuel truck exploded Wednesday outside a hotel where U.N. observers are staying, wounding at least three people, Syrian state TV reported. Activists also reported clashes in different parts of Syria, including clashes with rebels near the government headquarters and the Iranian embassy both in Damascus.

An activist based in the area said smoke was billowing from behind the Iranian embassy building, which is under construction. The present Iranian embassy building and the Cabinet headquarters are also nearby, he said.

“We heard shooting and explosions for a while,” said the activist Maath al-Shami.

Deputy Foreign Minister Faisal Mekdad toured the area of the blast and said none of the U.N. staff was hurt. The explosion occurred as U.N. humanitarian chief Valerie Amos was in the Syrian capital but her team is believed to be staying at a different hotel.

The blast was the latest in a series of explosions that have hit Damascus in the past months as clashes between government troops and rebels reached the capital, which had been relatively quiet since the uprising against Assad erupted in March last year.

Wednesday’s explosion went off about 300 meters (yards) from the military command. According to an Associated Press reporter at the scene, the blast was inside the parking lot of a military compound. The lot is near the Dama Rose Hotel, popular with the U.N. observers in Syria and where many of the mission staff are staying.

The hotel was slightly damaged, with some of its windows shattered. A Labor Union building across from the hotel was also damaged and black smoke was seen billowing high into the sky before the fire was extinguished. Several fire engines arrived shortly after to fight the blaze, which took less than an hour to put out.

U.N. officials in Damascus said there was an explosion near the hotel they used adding that no U.N. staffers were hurt.

“This is a criminal act that shows what kind of attacks Syria is being subjected to,” Mekdad, the deputy foreign minister, told reporters at the scene. “Such explosions will not affect Syria.”

“I confirm that we are with the U.N. and we will do all we can to guarantee their protection so that they carry out their role,” he added.

Activists say more than 20,000 people have been killed since the start of Syria’s revolt, inspired by other Arab Spring uprisings against autocratic regimes in the region. The conflict has slowly morphed into a full blown civil war.

It was not immediately clear who was behind Wednesday’s explosion or what was the intended target. There have been several high-profile bombings in the Syrian capital. On July 18, an explosion in a key government headquarters in Damascus killed four top generals, including Assad’s brother-in-law. And in March, a double suicide bombing in Damascus killed 27 people.

“Those who carry out such terrorist attacks are destroying their country in order to get some pounds,” shouted a Damascus resident, Ali Mohammed Ismail, 48, who said he happened to be in the area when the explosion went off.

In East Timor, U.N. Secretary-General Ban Ki-moon said he could not confirm whether a successor has been named for Syrian peace envoy Kofi Annan. On Tuesday, a spokesman for Annan said Syrian authorities have backed former Algerian Foreign Minister Lakhdar Brahimi as his successor, but it was unclear whether Brahimi had accepted the post.

Ban said the international community shares a “sense of collective responsibility” for the bloodshed and a growing humanitarian crisis within Syria and among refugees who fled to neighboring countries.

“How long do we have to endure this kind of tragedy?” he told a news conference in Dili. “This is not justice and this not acceptable. That is why now I am expediting the selection of successor of special envoy and we are trying our best effort to provide humanitarian assistance to more than one million Syrian people affected because of this situation.”

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AP writers Suzan Fraser in Ankara, Albert Aji in Damascus and Guido Goulart in Dili , East Timor contributed to this report.

Copyright 2012 The Associated Press.