Thousands of Syrian migrants have until Tuesday to leave Istanbul or face expulsion from Turkey’s biggest city.
Authorities have told unregistered migrants to return to the province they are registered in, as part of a bid to relieve pressure on the city.
But some Syrians told the BBC 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 order to relocate was issued in late July, giving those affected about a month to comply.
Since Syria’s civil war began over eight years ago, some 3.6 million Syrians have fled north to neighbouring Turkey, the BBC’s Mark Lowen reports.
About half a million are registered in Istanbul – but estimates suggest twice that number are living there, having travelled from the provinces they were first registered in.
Announcing the move last month, the governor of Istanbul said Syrians with the right to be in the city should carry their passports and identity documents with them at all times, and announced continuous checks at bus and train stations.
By early August some 12,000 migrants had been transferred to their registered province, and more than 2,600 unregistered people had been put into centres run by the interior ministry. Thousands of workplaces have been inspected “to prevent unregistered employment”.
Polling has shown a decline in support for Syrian refugees – from about 70% to 40% – seen as one factor that drove President Erdogan’s party out of power in Istanbul in this year’s mayoral election. Eight years on, doors in Turkey are closing, and the welcome is running out, our correspondent reports.
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.
But the BBC has spoken with Syrians in Turkey who say busloads of migrants are being transferred to Idlib – just across the border – where fighting is escalating.
On Monday, a Syrian government air strike 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.