Thousands of Syrian migrants have until Tuesday to leave Istanbul or face expulsion from Turkey's biggest city.

Axar.az reports citing BBC that authorities have told unregistered migrants to return to the province they are registered in, as part of a bid to relieve pressure on the capital.

But some Syrians told that many were being deported to Idlib, inside Syria, where fighting is escalating. They say many are being forced to sign voluntary return documents that they cannot read or understand.

The Turkish government says migrants who want to return to Syria are being transported to safe areas of the country under the control of the Turkish army. Syrians in Turkey say busloads of migrants are being transferred to Idlib - just across the border - where fighting is escalating.

On Monday, a Syrian government airstrike on a Turkish convoy killed three civilians and injured 12 more, Turkey said.

The region is supposed to be protected by a "buffer zone" agreed last year. But Syria's government has been stepping up its attacks in recent months.