Croatia says it will have to close its border with Serbia too, if another 7,000 or 8,000 migrants and refugees cross over in a single day.
Croatia has become the latest European country to feel the pressure of Europe's migrant crisis as thousands of refugees pour in.
The influx comes after Hungary sealed its border with Serbia, shutting off a major entry point into the European Union.
Around 9,000 migrants poured in from Serbia in a single day after Wednesday's clashes between migrants and refugees and Hungarian riot police.
Croatia has warned it has limited capacity for accepting and registering thousands of migrants arriving from Hungary.
Interior Minister Ranko Ostojic says the country would provide safe passage to reception centres around the capital, Zagreb.
But he says those not seeking asylum will be considered illegal immigrants.
"Our capacity is absolutely full, we are asking from them that all the countries on the way, on the route - that means Greece, Macedonia and Serbia - they have to respect all contracts, all memorandums, all conventions. They have the same obligations as Croatia to keep people under international protection in their reception camps."
Croatia is expecting an influx of more than 20,000 people over the next two weeks.
Earlier, crowds pushed through police lines in the eastern Croatia town of Torvarnik, with people trampling and falling on each other amid the chaos.
Adding to the country's burden, Slovenian police have said they'll return to Croatia some 150 migrants on a train halted on the Slovenian side of the border.
Francine Uenuma, a spokeswoman for aid agency Save the Children, is concerned for the migrants.
She says families migrating through the Balkans are determined to continue their journey along whichever route is open.
"Our concern is that it's an additional distance which means additional suffering for people who are determined to go. I think the extent that families are going to get to the next border shows you that's what's behind them is worse than what's in front of them if they're willing to continue to do this and continue to try."
The European Union has called an emergency summit next week to try to overcome the disarray.
Croatia, its newest member state, says it may have to use the army to stop illegal migrants criss-crossing the western Balkans in their quest for sanctuary in the 28-nation bloc.
The EU is split over how to handle the hundreds of thousands of people most of whom are fleeing war and poverty in Syria, Iraq, Afghanistan and Pakistan.
But the EU Migration Commissioner, Dimitris Avramopoulos, says he hopes the summit succeeds in convincing reluctant eastern European states to sign up to a quota system for the distribution of refugees.
"We are going to try again to convince the last ones who were not very willing to follow the same way, tell them that they - we - have to express our determination to stick together in a spirit of unity, work together in solidarity in order to prove that the (European) Union is here, determined and united."
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