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.
About half a million Syrians 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.
Doors in Turkey are closing
By Mark Lowen, BBC Istanbul Correspondent
Turkey is proud of its reputation as the world's largest host of refugees, welcoming 3.6 million Syrians since the war began. But patience with the Syrians is wearing thin.
Under international humanitarian law, the "non-refoulement" principle prohibits deporting migrants to a warzone. The Turkish government says Syrians returning home are going voluntarily, and to areas secured by the Turkish army.
But we've spoken to several who say they've been deported to Idlib, where fighting rages. In Syrian neighbourhoods of Istanbul, stories of forced relocation are everywhere; of people made to sign voluntary return documents they weren't given the chance to read.
"When I crossed the border and saw the Turkish flag behind me, I realised I hated it because this is a racist country," said one Syrian, who had been deported but had smuggled himself back to Istanbul. Another, who we spoke to in Syria, said Turks saw Syrians "like insects sucking their blood".
Eight years since the war began, doors are closing in Turkey and the welcome is running out.
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.
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.
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.