BEIRUT: The Syrian regime is expanding its use of widely banned cluster bombs, an international human rights group said Saturday as the deadlocked conflict entered its third year.

In new violence, rebels detonated a powerful car bomb outside a high-rise building in the eastern city of Deir el-Zour, setting off clashes with regime troops, state TV and activists said.

The blast came a day after Syrians marked the second anniversary of their uprising against President Bashar Assad. The rebellion had begun with largely peaceful protests but in response to a regime crackdown turned into an insurgency and then a civil war.

In recent months, the regime has escalated airstrikes and artillery attacks on rebel-held areas in the north and east of the country, rights groups have said.

On Saturday, the New York-based Human Rights Watch said Syrian forces have dropped at least 156 cluster bombs in 119 locations across the country in the past six months, causing mounting civilian casualties.

Two strikes in the past two weeks killed 11 civilians, including two women and five children, the report said. The group said it based its findings on field investigations and analysis of more than 450 amateur videos.

Cluster bombs open in flight, scattering smaller bomblets. They pose a threat to civilians long afterwards since many don't explode immediately. Most countries have banned their use.

A senior Syrian government official denied Saturday that regime forces use cluster bombs and said, "Many amateur videos are doubtful."

He spoke on condition of anonymity because he was not authorized to make official statements to the media.

The fighting in Syria has killed some 70,000 people and displaced 4 million of the country's 22 million people, according to U.N. estimates.

The conflict remains deadlocked, despite some recent military gains by the rebels.

On Saturday, rebels in Deir el-Zour detonated a car rigged with more than two tons of explosives next to the tallest building in the city, known as the Insurance Building, state TV said.

The TV said rebels entered the building after the blast but were pushed out by government forces.

The Britain-based Syrian Observatory for Human Rights, an activist group, also reported clashes between rebels and regime troops following the explosion. Regime forces also shelled several areas of the city, the group said.

In an amateur video said to be showing Deir el-Zour, heavy gunfire was heard in the background and a cloud of smoke was visible.

Late Friday, rebel fighters from the al-Qaida-linked group Jabhat al-Nusra and other Islamist factions seized a military base and munitions depot in the town of Khan Touman in the northern province of Aleppo, the Observatory said.

It quoted witnesses as saying rebel fighters drove off with truckloads of ammunitions and weapons. The Khan Touman base is only a few kilometers (miles) from a military engineering academy that is considered a key government stronghold in the province, the Observatory said.

Despite rebel advances, Assad has been digging in, particularly in the densely populated western part of the country. He has armed and mobilized loyalists, and repelled rebel attacks on his seat of power, the capital Damascus.

The rebels have appealed to the West for military aid, including anti-aircraft weapons, to help them break the stalemate.

On Friday, a European Union summit heard an appeal by Britain and France to lift the EU ban on arming the rebels.

The 27 national leaders were unable to reach a consensus and asked their foreign ministers, who will meet late next week in Dublin, to try to hash out a common position.

Samir Nashar, a member of the Syrian National Coalition, the main opposition group in exile, said he hoped France and Britain would defy the EU if the embargo remains in place.

"I prefer that there is a consensus and a joint resolution," he said Friday in Istanbul. "But if there's no consensus, I still think France and Britain will act unilaterally."

The French foreign minister suggested earlier this week that his country might arm the rebels even if the EU disagrees.