The Former Yugoslav Republic of Macedonia has reportedly closed its southern frontier with Greece to Afghan migrants.
Only Iraqis and Syrians are apparently being allowed through.
Greek police say it is because Serbia has made the same decision and officials fear migrants will get stuck in Macedonia.
“Everyday the Syrian and Iraqi people, about one hundred, two hundred, they cross this border they don’t let Afghans cross from this border,” said Masoud Jahangirg, an Afghan refugee.
Police started restricting the flow of migrants across the border on Saturday, conducting body searches and demanding passports.
It has all led to a build-up of migrants waiting on the Greek side of the frontier.
According to police, 1,000 people were stranded on Sunday and a further 4,000 were waiting in buses closeby.
A Macedonian police spokesperson has denied there is a bar on Afghan migrants, blaming Serbia instead.
Meanwhile, Austria is coming under renewed fire after capping the number of migrants who can claim asylum to 80 a day, while allowing 3,200 to transit through.
German Interior Minister Thomas de Maiziere says the move is “unacceptable” and sends out the “wrong signal.”
Berlin fears those crossing the border are heading straight for Germany, which took in more than a million migrants last year.