Macedonian authorities confirmed earlier reports by Greek police, saying they are responding to a "domino effect" of similar restrictions by their neighbors to the north.
"The Serbian side has stopped the entrance for Afghan refugees," Macedonian police spokeswoman Natalija Spirova Kordikj told The Associated Press, adding that there is no official explanation from the Serbian side for the move.
Macedonian police started restricting the flow of migrants across the Greek-Macedonian border Saturday, conducting body searches and demanding passports. Earlier, they had accepted Greek police's official documents attesting that an individual had been processed.
The moves have led to a buildup of migrants waiting at the Greek side of the border. Greek police said 800 were stranded at the border Sunday and another 2,750 were waiting in 55 buses nearby.
In the 24 hours to 6 a.m. local (0400 GMT) Sunday, only 310 migrants had been allowed into Macedonia.
At the collection center in Tabanovce, on the Macedonian side of the border with Serbia, a total of 617 Afghan refugees are waiting to be allowed into Serbia, Kordikj said.
"367 have been waiting since Friday. Another 250 arrived at the collection center (Saturday)," she said.