Syrian government forces have renewed their ground and aerial bombardment of the city of Aleppo, in what the United States says it fears will become a massacre.
A State Department spokesperson, Victoria Nuland, says there are credible reports of tank columns moving towards the city as well as air strikes from helicopters and fighter planes.
She says the signs are ominous.
Pro-regime media are warning of a "mother of all battles" for Aleppo, which has more than two million residents.
Rebels had earlier been reported to be stockpiling ammunition and medical supplies in preparation for an all-out offensive by government forces.
Rebel fighters were shown in unverified footage celebrating after overrunning a police station in the south of the city.
There have been further heavy clashes between the two sides in a number of districts, according to the BBC.
Reports from inside the city say government artillery and mortar rounds have been fired as helicopter gunships flew over a number of areas, sporadically shelling targets.
Black plumes of smoke are said to have been rising in western areas, accompanied by the sound of loud blasts.
The armed opposition claims to control more than half of Aleppo.