Serbian security forces have increased patrols and deployed elite units in an effort to stop thousands of migrants from crossing its border into Hungary.
Serbian officials say some 1,000 asylum seekers are caught each day by Hungarian or Serbian border guards trying to illegally cross the border.
The large majority of the migrants are citizens of Kosovo, although there are also Syrians, Afghans, Iraqis, and others.
Hungarian officials say almost 23,000 illegal migrants came to Hungary in the first six weeks of 2015, compared to a total of some 43,000 in all of 2014.
Kosovar officials say about 100,000 Kosovar Albanians have left the country since summer 2014.
Most of the migrants go from Hungary to wealthier EU countries such as Germany, Austria, and France.
Kosovo, one of the poorest countries in Europe, declared independence from Serbia in 2008.