"With the build-up of heavy weapons around Aleppo, Assad is preparing to carry out a fresh slaughter of his own people," foreign ministry spokesman Bernard Valero told AFP, echoing similar concerns expressed by US officials.

The US State Department on Thursday warned that Assad may be preparing to carry out a massacre following "credible reports" that columns of tanks, helicopter gunships and warplanes were being readied for an assault on the city.

Valero said France shared US concerns over the rapidly deteriorating situation in Aleppo and called on Assad to end the violence and step down.

"Our message is that Assad must go," he said.

Aleppo, Syria's most populous city and also the country's commercial hub, has been the theatre of fierce fighting between the opposition and Assad's forces for the last week.