GENEVA — Croatia, Macedonia and Serbia have started screening migrants and refugees on the basis of nationality and allowing entry only to those from Afghanistan, Iraq or Syria, the United Nations refugee agency said on Thursday.

Serbia sent 200 people who were not from those three countries back to Macedonia on Tuesday night, William Spindler, a spokesman for the refugee agency, said in an interview.

Image Border guards checked the documents of a refugee in Idomeni, Greece, before letting him crossing into Macedonia. Credit... Mauricio Lima for The New York Times

Some of those who were sent back were admitted to a refugee reception post in Tabanovce, Macedonia, but about 100 were refused entry and are now stranded in the open, he said.