A Syrian fireman is seen after a bomb exploded at a military site near a hotel used by United Nations monitors in Damascus

REUTERS

A Syrian air raid killed at least 30 people in a rebel-held northern border town on Wednesday, opposition activists said, and a bomb went off near U.N. and military sites in the capital Damascus, wounding three.

As the violence intensified, U.N. human rights investigators accused forces loyal to Syrian President Bashar al-Assad of committing war crimes and crimes against humanity, reports Reuters.

They said rebels had also committed war crimes, but the violations "did not reach the gravity, frequency and scale" of those by state forces and the pro-Assad shabbiha militia.

"The commission found reasonable grounds to believe that government forces and the shabbiha had committed the crimes against humanity of murder and of torture, war crimes and gross violations of international human rights law and international humanitarian law, including unlawful killing, torture, arbitrary arrest and detention, sexual violence, indiscriminate attack, pillaging and destruction of property," said the 102-page report by the independent investigators led by Paulo Pinheiro.

Residents in the northern border town of Azaz screamed and shouted "God is greatest" as they carried bloodied bodies from collapsed concrete buildings, video posted by activists showed.

The opposition Syrian Observatory for Human Rights said dozens had been killed. One activist in the town said at least 30 bodies had been found and rescuers were searching for more.

Video footage, which could not be verified, showed crowds of residents wrestling with steel bars and pulling away a giant slab of concrete to reveal the dust-covered arm of a child.

"This is a real catastrophe," said an activist who gave his name only as Anwar. "An entire street was destroyed."

Assad's forces have increasingly used helicopter gunships and warplanes against the lightly-armed insurgents.

In Damascus, a bomb exploded in the car park of a hotel used by U.N. monitors, but several military buildings are also in the vicinity and it was not clear what the target was.

No U.N. staff were hurt in the blast, which occurred exactly four weeks after a bomb killed four of Assad's senior aides. The bomb set a fuel tanker ablaze and black smoke billowed over the city. Ash and dust covered white U.N. vehicles parked nearby.

State media said three people were wounded in the bombing and several rebels were killed or captured in a separate gunbattle with security forces in the western district of Mezze.

Deputy Foreign Minister Faisal Mekdad said the bomb blast proved "the criminal and barbaric nature of those who carry out these attacks - and their backers in Syria and abroad".