Tensions are building among thousands of migrants heading north through Balkan states as their route to Western Europe is stalled by new controls.
Croatia had asked its northern neighbour Slovenia to accept 5,000 migrants daily, but Slovenia said it would only take half that number.
The move has led to a build-up of people on Croatia's border with Serbia.
An official told the BBC that Croatia could run out of room in its transit camps within days.
Buses crammed with people were backed up in Serbia on Sunday, and tempers flared between frustrated migrants and overstretched police officers.
Overnight, many were forced to wait in the cold and the rain.
Thousands of asylum seekers are travelling north through the Balkans, with most aiming to reach Austria, Germany and other EU states.
Hungary, citing security concerns, has closed its borders with Serbia and Croatia, forcing migrants to switch to a slower route via Slovenia.
Explaining Slovenia's new restrictions on Sunday, Interior Ministry State Secretary Bostjan Sefic said its northern neighbour Austria was only accepting a maximum of 1,500 people a day.
He said that Slovenia "cannot accept unlimited numbers of migrants if we know that they cannot continue their journey".
Image copyrightAP
Image copyrightAP
Image copyrightAP
Image copyrightAP"If we would accept 5,000 migrants per day that would mean 35,000 would be in Slovenia in 10 days," he said. "That would be unacceptable."
About 5,000 migrants spent a cold night in a transit camp in Opatovac, eastern Croatia.
Over the border in Serbia, thousands had been kept in about 50 buses since early on Sunday waiting to cross to Croatia.
"We are waiting here four hours on the bus," said Muhammad Samin from Afghanistan. "The weather is too cold. We wear lots of shirts. The children are also in the cold. No food."
The migrants - many from Afghanistan, Syria and Iraq - have already spent weeks walking from Turkey, via Greece, Macedonia and Serbia.
