More than 80 people have reportedly been killed in intense aerial attacks by government forces on the northwest Syrian city of Aleppo and nearby towns.
President Bashar al-Assad's forces' nine-day bombardment of rebel-held areas of Aleppo, Syria's biggest city, has killed more than 300 people, as the government steps up its 18-month battle with rebel fighters in the region.
The Syrian Observatory for Human Rights said on Monday that many of the casualties, including scores of women and children, were killed by so-called barrel bombs dropped from helicopters.
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 December 15.
On Monday, 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 written statement, the White House called on the Syrian government to "fulfil 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.
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," said Ole Solvang, the group's senior emergency researcher, 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 December 4 they fired at least 10 surface-to-surface rockets into residential areas, killing at least 19 civilians.
The army is thought unlikely to be able to recapture major parts of Aleppo before the talks in Switzerland start on January 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 two million people to seek refuge abroad and driven more than a third of the country's 23 million people from their homes.