Croatia has closed seven of the country's eight border crossings with Serbia "until further notice" following a massive influx of migrants and refugees, the interior ministry said.
"Traffic is banned on the border crossings of Tovarnik, Ilok, Ilok 2, Principovac, Principovac 2, Batina and Erdut," the ministry said on Thursday (local time).
More than 11,000 migrants have entered Croatia from Serbia since early on Wednesday, the ministry added.
The migrants began heading for Croatia after Hungary sealed its border with Serbia earlier this week, cutting off a key route into the EU used this year by more than 200,000 migrants, many of them fleeing violence in the Middle East and Afghanistan.
Croatia had said it would let people pass through freely on their way to other European Union countries, but Prime Minister Zoran Milanovic warned his country's resources for dealing with the influx were "limited".
Piling on the pressure, Slovenia announced late on Thursday that it had stopped a train from Croatia carrying migrants, saying some 150 passengers would be returned to Zagreb because they did not have the necessary documents for onward travel.
Slovenia later suspended all train traffic between Slovenia and Croatia until Friday morning, Slovenian news agency STA reported.