More than 300 people have been killed in a week of air raids on the northern Syrian city of Aleppo and nearby towns by President Bashar al-Assad's forces, a monitoring group said on Monday.

Many of the casualties, who included scores of women and children, were killed by so-called barrel bombs dropped from helicopters, the Syrian Observatory for Human Rights said.

Syrian authorities say they are battling rebels who have controlled parts of Aleppo, Syria's biggest city, and most of the surrounding countryside for the past 18 months.

But human rights group have condemned the use of the improvised bombs - oil drums or cylinders which are packed with explosives and metal fragments, often rolled out of the aircraft cargo bay - as an indiscriminate form of bombardment.

Rami Abdulrahman, director of the British-based pro-opposition Observatory, said 87 children and 30 women were among the 301 people killed in the Aleppo air raids since Dec. 15.

The US condemned "the ongoing air assault by Syrian government forces on civilians, including the indiscriminate use of SCUD missiles and barrel bombs in and around Aleppo over the last week."

In a statement, the White House also called on the Syrian government to "fulfill its November commitment to do more to facilitate the safe and unhindered delivery of humanitarian assistance, so that millions of Syrian men, women, and children have access to urgently needed services."

Assad's forces have clawed back territory to the southeast of the city in recent weeks and reasserted control over several Damascus suburbs in the build-up to planned peace talks next month aimed ending Syria's almost three-year-old conflict.

The army is thought unlikely to be able to recapture major parts of Aleppo before the talks in Switzerland start on Jan. 22, but Abdulrahman said the air raids might be aimed instead at turning the remaining residents against the rebel fighters by showing that the insurgents could not protect civilians.

The pro-opposition Observatory said more than 125,000 people have been killed in Syria's civil war, which has also forced 2 million people to seek refuge abroad and driven more than a third of the country's 23 million people from their homes.

Human Rights Watch said in a report at the weekend that barrel bomb attacks in Aleppo had hit residential and shopping areas, describing the air raids as illegal.

"The Syrian air force is either criminally incompetent, doesn't care whether it kills scores of civilians, or deliberately targets civilian areas," the group's senior emergency researcher, Ole Solvang, said in the report.

Rebels also appeared to have violated international law by indiscriminately launching rockets and mortar bombs at civilian areas in the government-controlled part of Aleppo, Human Rights Watch said.

It said on Dec. 4 they fired at least 10 surface-to-surface rockets into residential areas, killing at least 19 civilians. – Reuters