Croatia has barred Serbian citizens from entering the country after Serbia banned Croatian cargo traffic in a growing dispute over the huge flow of migrants across their joint border.
"Serbian passport holders and cars registered in Serbia cannot enter Croatia until further notice," a police officer told Reuters by telephone from Bajakovo, the main crossing point between the two ex-Yugoslav republics.
Serbia banned Croatian cargo traffic and goods late on Wednesday, helping plunge bilateral relations to their lowest point since the overthrow of late Serbian leader Slobodan Milosevic in 2000.
Belgrade imposed its embargo in retaliation for border restrictions levied by European Union member Croatia, which has hit out at its eastern neighbour for directing the flow of migrants coming up through the Balkan peninsula over their joint border.
Zagreb had banned all trucks but those carrying perishable goods from entering the country from Serbia and shut seven of eight road border crossings, saying Serbia should direct the migrants to Hungary and Romania too.
"I am sorry about this. We had planned to open the border today but now we have to react to this," Croatian Prime Minister Zoran Milanovic, whose centre-left cabinet faces a parliamentary election this year, said after Serbia imposed the ban.
"There will be no war or violence, everything will be calm, but this is not normal behaviour (by Serbia)," Milanovic said on television from Brussels, where he was attending a summit of EU leaders.
More than 40,000 migrants, many of them Syrian refugees, have entered Croatia from Serbia since Tuesday last week when Hungary, both countries' neighbour to the north, barred their entry to the EU by sealing its southern border with Serbia with a metal fence.
Both Serbia and Croatia were part of communist Yugoslavia but Croatia broke away in 1991 and fought a war against Belgrade-backed Serb rebels until 1995. Croatia joined the EU in 2013, and Serbia wants to follow suit.
Serbia has been bussing migrants straight to the Croatian border after they enter Serbia from Macedonia.
Croatia is sending migrants north across its own border with Hungary, which in turn sends them to Austria, but Zagreb says it cannot cope with the numbers and wants Serbia to send some of the migrants to Hungary and Romania.
"I cannot allow that the people who travel in an uncontrolled way become organised at the end of their journey, that's when the Serbian authorities come in, and end up at Croatia's border."
The escalating row comes as European Commission Vice President Frans Timmermans warned that unless the migrant crisis in Europe was addressed properly there would be a surge of right-wing extremism across the continent.
Mr Timmermans also said that there needed to be better protection of the European Union's borders to deal with the hundreds of thousands of migrants fleeing war and poverty in the Middle East, Asia and Africa.
"We have to make sure that those countries where people arrive are better placed to make sure people are registered, that people who don't have the right to asylum are returned swiftly," he told BBC radio.
"If we're not able to tackle this issue, if we're not able to find sustainable solutions, you will see a surge of the extreme right across the European continent."
His comments come after the EU leaders pledged at least €1 billion for Syrian refugees in the Middle East and closer cooperation to stem the migrant flows.