Slovenian authorities say they have refused to let in more than 1,000 migrants arriving from Croatia, after a daily quota had been reached.
A train carrying around 1,800 people arrived overnight but only 500 of "the most vulnerable" were allowed into Slovenia, police spokesman Bojan Kitel said on October 19.
Croatia had asked its northern border to accept 5,000 migrants daily but Slovenia insists it would only take half that number.
The move has led to a build-up of migrants in Croatia and Serbia.
The UN refugee agency said on October 19 that more than 10,000 migrants are currently stranded in Serbia.
UNHCR spokeswoman Melita Sunjic warned of shortage in aid for the migrants stranded at the Serbian-Croatian border: "There is a lack of food, lack of blankets, we are missing everything."
Tens of thousands of migrants -- mostly fleeing violence in Syria, Afghanistan, and Iraq -- are travelling north through the Balkans, aiming to reach Western Europe.
The migrants switched to a route via Slovenia when Hungary closed its border with Croatia and Serbia.