By Branko Filipovic and Aleksandar Vasovic

BATROVCI, Serbia/OPATOVAC, Croatia (Reuters) - Serbia threatened to retaliate on Tuesday after Croatia halted all cargo traffic from its eastern neighbor, with relations between the ex-Yugoslav republics starting to fray over the flow of migrants across their joint border.

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

They are being bussed by Serbia direct to the Croatian border, having entered Serbia from Macedonia. Croatia says it cannot cope with the numbers and, in apparent retaliation, has closed all but one of its road border crossings with Serbia and halted trucks from entering on the Belgrade-Zagreb highway, one of Serbia’s two main arteries to the west.

“We’re not fools; we see what they’re doing,” Croatian Prime Minister Zoran Milanovic told reporters.

Both Serbia and Croatia were part of socialist Yugoslavia but Croatia broke away in 1991 and fought a war against Belgrade-backed Serb rebels until 1995.

With a queue of trucks on the highway 12 kilometers (7 miles) long, Serbia called an extraordinary cabinet session to discuss possible ‘counter-measures’.

“We’re waiting for the EU to react by 2 p.m. (8:00 a.m. EDT), otherwise Serbia will respond calmly, without violating any rules, but it will demonstrate that Croatia cannot torture or humiliate Serbia and destroy Serbian business without consequences,” Serbian Prime Minister Aleksandar Vucic said on Tuesday.

EU MEETING

Croatia is sending migrants north across its own border with Hungary – which in turn sends them to Austria – but is struggling to keep pace with the influx. It says Serbia should direct them to Hungary and Romania too.

A new migrant crossing point emerged overnight at the Croatian village of Bapska, with hundreds seen walking through rolling hills to a new camp for 4,000 at Opatovac, which is fast reaching capacity.

Story continues

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.

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

Ostojic, who was due to attend a meeting of EU interior ministers in Brussels on Tuesday to agree responses to the migration crisis, said Croatia was “absolutely ready” to shut, to all traffic, the main Bajakovo-Batrovci crossing on the highway, the last crossing that remains open.

Serbia’s Vucic threatened to retaliate.

If Croatia does not lift the blockade, he 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.

(Additional reporting by Aleksandar Vasovic in OPATOVAC, Croatia and Igor Ilic and Zoran Radosavljevic in ZAGREB; Writing by Matt Robinson; Editing by Ralph Boulton)