Syrian opposition activists say government forces are continuing a ground and aerial bombardment of the nation's largest city, Aleppo, in what the United States says it fears could become a massacre.

The Britain-based Syrian Observatory for Human Rights said President Bashar al-Assad's forces were firing machineguns from helicopters in several Aleppo neighborhoods Friday. It reported clashes and explosions in other areas of the city, as well as several deaths.

The group also reported deadly government attacks in Dera'a province and said violent clashes were taking place between troops and rebels in the city of Idlib.

Aleppo had been relatively quiet until two weeks ago when rebels launched an operation to seize control.

State Department spokeswoman Victoria Nuland said Thursday that there were columns of tanks outside Aleppo that seemed to be massing for an attack. She said it appears the Syrian government is preparing for a massacre.