The UN refugee agency criticised Turkey on Friday for sending home at least 130 Syrians without its scrutiny and urged it to investigate the riot which sparked the departures that some witnesses said were forced.

Turkey denied on Thursday it had rounded up and deported hundreds of Syrian refugees following unrest at the Suleymansah border camp, highlighting the strain the exodus from Syria's civil war is placing on neighbouring states.

The Geneva-based United Nations agency reiterated the principle that forced returns violate international law and said they could not be used as a "punishment or deterrent".

"UNHCR was not invited by authorities during the return process to monitor the procedures. Persons under international protection who have violated the law of the host country are subject to the relevant national laws and judicial procedures," Melissa Fleming, chief spokeswoman of the UN High Commissioner for Refugees, said in remarks emailed to Reuters overnight.

"Return to the country of origin, even voluntarily, is also subject to standards and procedures where individuals may be placed at risk on return," she said.

The refugees returned to areas of northern Syria held by rebels fighting President Bashar al-Assad. UNHCR has no direct access to the area and does not know what happened to them.

Two Syrian refugees still in the camp and a camp official have said the refugees were forcibly deported. Turkish media reports said the protest began after a boy died in a tent fire blamed on an electrical fault.

Witnesses said hundreds of Syrians were bussed to the border after clashes on Wednesday in which refugees in the Suleymansah camp, near the Turkish town of Akcakale, threw rocks at military police, who fired teargas and water cannon.

"UNHCR would encourage authorities to assess any underlying issues which may have led to the incident which erupted in the Akcakale camp on 27 March, and where necessary to consider launching a review or as needed an investigation," Fleming said.

Turkey's Foreign Ministry said 130 people, identified on camera as being "involved in the provocations", returned to Syria voluntarily, either because they did not want to face prosecution or because of repercussions from other refugees.

"Reports that this group was expelled across the border are incorrect," a Foreign Ministry statement said.

"As required by the temporary protection status and within the framework of the 'open door' and 'non-refoulement' principle, our country does not turn back Syrians wanting to come to Turkey or forcibly evict those in our country," it said.

More than 1.2 million Syrians fleeing violence and persecution have registered as refugees or await processing in neighbouring countries and North Africa, the UNHCR says. They include 261,635 in Turkey, mostly staying in 17 camps.

The UNHCR noted "the high standard of assistance and protection extended to Syrians hosted in Turkey" and commended its government and people "for their ongoing generosity and sheltering of those in need", Fleming said.-Reuters