The officials, speaking on condition of anonymity, said thousands of Iranian troops had arrived to take part in the ground offensives in support of President Bashar al-Assad.

A big government offensive in the area near the Turkish border is likely to further anger NATO member Ankara, which has backed insurgents fighting Assad and has already expressed deep concern over Russian air strikes that have targeted them.

Control of Aleppo city and the surrounding province is divided among the Syrian government, a range of insurgent groups fighting Assad, and the Islamic State group that controls some rural areas near the city.

“The big battle preparations in that area are clear,” said one of the officials. “There is a large mobilization of the Syrian army … elite Hezbollah fighters, and thousands of Iranians who arrived in stages in recent days,” said the official.