The forces of Bashar al-Assad, the Syrian president, are expanding their use of widely banned cluster bombs, causing "mounting civilian casualties," an international human rights group said.

Saturday's report by Human Rights Watch comes just after the conflict reached the two-year mark earlier this week.

The New York-based group said Syrian government forces have dropped at least 156 cluster bombs in 119 locations across the country in the past six months, causing mounting civilian casualties.

"Syria is expanding its relentless use of cluster munitions, a banned weapon, and civilians are paying the price with their lives and limbs," said Steve Goose, HRW director of the arms division.

"The initial toll is only the beginning because cluster munitions often leave unexploded bomblets that kill and maim long afterward," Goose said.

"All nations that have joined the treaty banning cluster bombs have a legal obligation to speak out and condemn Syria's ever-expanding use of these indiscriminate weapons, yet too few are doing so," he added.

In the past two weeks, two strikes 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.

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

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

Deir el-Zour

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

The TV said opposition fighters 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 the fighters and government troops following the explosion.

Assad 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.