Serbia threatened to retaliate after neighbour Croatia halted all cargo traffic from Serbia following an influx of migrants over their joint border.

More than 30,000 migrants, many of them Syrian refugees, have entered Croatia from Serbia since Tuesday last week, when Hungary barred their entry to the European Union over its southern border with Serbia with a metal fence.

Croatia has said it cannot cope with the numbers; a new crossing point emerged overnight at the Croatian village of Bapska, with hundreds seen walking through rolling hills to a camp Croatia has opened for migrants while authorities organise their transport north to Hungary or west to Slovenia.

Several thousand more were waiting in no-man's land between Sid in Serbia and Tovarnik in Croatia; some spent the night sleeping in a cemetery.

Croatia has already closed all but one road border crossing with Serbia and on early Monday halted trucks from entering on the highway between Belgrade and Zagreb, one of Serbia's two main arteries to the west. The blockage created a 12-km (7-mile) queue of trucks on the Serbian side.

The Croatian government has cited security reasons but local media say Zagreb is trying to put pressure on Belgrade to slow the flow of migrants over their border and send more north to Hungary or east to Romania.

Croatian Interior Minister Ranko Ostojic told RTL television late on Monday: "I think it is perfectly clear that what we are seeing is organised transport of migrants directly to Croatia (from Serbia)."

The Serbian government has called an emergency session for Tuesday afternoon to discuss possible counter-measures.

"There is no rational, logical explanation for stopping the flow of goods, of cargo traffic, on one of the main arteries," Serbian Justice Minister Nikola Selakovic told Serbian state television. "Closing that route is inflicting millions in damage to Serbian business and the state every day," he said.

If Croatia does not lift the blockade, Serbian Prime Minister Aleksandar Vucic said late on Monday, "due to violations of numerous bilateral and international agreements we will be forced to protect our country by legal means."

"That will hit their country far harder than their measures are hurting ours," he said.