An increasing number of legally-registered migrants are being illegally deported from Serbia to Bulgaria and Macedonia.
Some 1,000 people from the Middle East, Asia and Africa "were expelled in November alone along the Balkans route... more than before", the UNHCR spokeswoman in Serbia, Mirjana Milenkovska, told AFP.
Hundreds of thousands of migrants took the Balkans route through Macedonia and Serbia, Croatia and Hungary as they tried to reach western Europe from Greece before it was largely shut down in March.
Non-governmental human rights organisations and activists also warned that an increasing number of legally-registered migrants were being "illegally deported" from Serbia to Bulgaria and Macedonia.
On December 17 "a seven-member Syrian family, including a two-year old child, 16-year old girl and two women almost froze after being left in a forest in -11 Celsius degrees (12.2 Fahrenheit) to walk towards Bulgaria some one kilometre (half a mile) away," an activist said.
"The family was registered in Belgrade and on its way to a refugee centre, when they were taken off the bus by a police or army unit that tore their documents," said Gordan Paunovic of the Info Park group that provides help to refugees.
They were eventually rescued by a regular police officer from a nearby town who was alerted by activists, Paunovic said.
The Belgrade Centre for Human Rights (BCHR) that represents the family said that the bus driver confirmed the information.
Serbian Defence Minister Zoran Djordjevic denied army involvement in the incident.
Some 7,000 migrants are currently in Serbia. Most are lodged in 13 official reception centres, while some 1,000 are sleeping rough in downtown Belgrade, Milenkovska said.
Some 109 people were reportedly deported in November to Macedonia from Serbia's border migrant transit centre at Presevo.
"They all came back to Belgrade and told us the same story: they were awakened at 4 am, put in a police van and than driven to a field in the middle of a forest on the border with Macedonia," Paunovic said.
Members of Macedonian Youth Lawyers Association (MYLA), who provide legal help to migrants in Macedonia, also "have recorded cases of persons who have been pushed back to Macedonia, although they had asylum papers from Serbia."