BLACE, Macedonia (AP) _ Serb forces ordered a train packed with hundreds of ethnic Albanians to return to Kosovo today after refusing to allow them to cross into Macedonia, international aid officials said.

A train with four or five cars, each crammed with 200 to 300 people, arrived early today at Djeneral Jankovic, the last station on the Yugoslav side of the border with Macedonia, said Ron Redmond, spokesman for the United Nations High Commissioner for Refugees.

``This is quite a worrisome development,'' Redmond said.

The train was full when it departed Kosovo Polje, near the provincial capital of Pristina. When it reached the town of Urosevac, 30 miles from the border, hundreds more crammed aboard, Redmond said, based on information from ten elderly men were allowed to cross into Macedonia.

``We are not sure why the Serbs are not allowing people off,'' Redmond said. ``They can turn this off and on like a tap.''

At midday, a series of explosions were heard on the Kosovo side of the border, but no information was available on the source of the explosions.

One person who managed to cross the border today was Besa Pajaziti, 23, a Pristina university student. She said her parents, who earlier had fled the city, arranged for a private car to drive her to Macedonia.

Residents in Pristina don't have enough to eat and the city was full of Serb police, who had taken over the library and civilian homes, she said. ``I can just say it's hell.''

NATO's air war against Yugoslavia began March 24. About 790,000 ethnic Albanians have since been expelled by Serb forces or fled Kosovo.