Migrant crisis: Balkans to limit intake
Balkan countries will only open their borders to migrants fleeing the Middle East and Afghanistan, the UN says.
Macedonia and Serbia made the move after Slovenia informed them it would not accept "economic migrants", country officials said.
Hundreds of people are now stranded on borders.
Right-wing European politicians called for border closures following indications that one of the Paris attackers posed as a migrant.
Serbia implemented the move late on Wednesday, according to a spokeswoman for the UN refugee agency UNHCR, Melita Sunjic.
"As of 6pm yesterday evening, Serbia started turning back (to Macedonia) all but Syrians, Iraqis and Afghans," she said.
A police spokesman in Slovenia confirmed the country would return "economic migrants" entering through Croatia, only allowing entry to those "from countries where there are armed battles".
The restrictions come after a Syrian passport - yet to be verified - was found next to the body of one of the perpetrators involved in Friday's attacks in Paris.
The passport was reportedly registered in the Greek island of Leros, and later in Serbia and Croatia.
Around 2,000 are now waiting to cross the border from Greece north into Macedonia, according to AFP.
The restrictions are likely to affect migrants from Eritrea which, after Syria, has the second largest number of people looking to move to Europe. The UN estimates that around 4,000 people leave Eritrea every month.
The Eritrean government says those who leave are economic migrants - but many claim to be fleeing military service.