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TOVARNIK, Croatia — Traffic began flowing through the major Serbia-Croatia border crossing at Bajakovo on Wednesday after a two-day blockade imposed to stem the flow of migrants across the two ex-Yugoslav republics' frontier.

The reopening should help ease tensions between the two former Balkan war adversaries that flared up as they struggled to cope with a steady flow of migrants, transiting the region en route to northern and western Europe in the past week.

"Trucks have started moving across the border at Bajakovo," Croatian state radio said in the late afternoon, after earlier reporting that cars and buses were allowed through.

A witness at the border said trucks had started moving from Croatia into Serbia, though not yet the other way round.

Croatia imposed the Bajakovo blockade for cargo on Monday in retaliation for Serbia directing migrants across the border. It earlier closed seven of eight border crossings to traffic.

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Serbia had set a Wednesday midnight deadline for Croatia to lift the blockade, threatening to retaliate with unspecified measures. Croatia joined the European Union in 2013 and Serbia wants to join the bloc.

Croatian authorities had agreed on Tuesday to allow in trucks with perishable goods from Serbia but their drivers refused to budge until all traffic resumed.

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 EU by sealing 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, and trekking through fields beyond the official border crossings. Croatia says it cannot cope with the numbers, saying Serbia should send them to Hungary and Romania too.

With a queue of trucks on the Belgrade-Zagreb highway 12 km (7 miles) long by Tuesday evening, Serbia gave Croatia until the end of Wednesday to lift the cargo blockade or face political, legal and economic retaliation.

“Serbia must reply to the destruction of its economic integrity and national policy,” Serbian Prime Minister Aleksandar Vucic said on Tuesday.

Croatian authorities later opened the nearby Tovarnik-Sid crossing to passenger traffic, but not trucks.

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.

A camp opened in Opatovac in eastern Croatia is fast reaching capacity, while thousands are stuck in no-man’s land between Sid and Tovarnik, some sleeping in a cemetery.