Croatian Prime Minister Zoran Milanovic has warned his country has limited capacity for accepting and registering thousands of migrants arriving from Hungary.
"We will be very constructive and co-operative (regarding migrants), but emphasise that there are limits to our capacities," Milanovic told a cabinet session on Thursday (local time).
Croatia is expecting an influx of more than 20,000 migrants over the next two weeks, a minister said on Thursday.
"We expect a peak within the next two weeks ... over 20,000 migrants," Health Minister Sinisa Varga said, quoted by the state-run HINA news agency.
The interior ministry said on Thursday 6200 migrants had arrived in the EU member state from Serbia in a little more than 24 hours.
Between 4000 and 5000 migrants attempted to board trains to the Croatian capital Zagreb from Tovarnik after crossing the border with Serbia, the UN refugee agency said.
"There are between 4000 and 5000 people here," Jan Kapic, a UNHCR spokesman, said from the small town in eastern Croatia on Thursday.
"Trains are coming but they can't take all these people."
Up to 5000 people arrived at the tiny train station overnight. The station was overwhelmed and people slept along the side of the tracks.
Police were lined up nervously along the track.
When a rumour spread that a train was coming, thousands of migrants leapt to their feet, pressing against the rail line. It proved a false alarm and everyone sat down again.
The migrants have entered Croatia in the past 24 hours, after Hungary effectively sealed its border, shutting off a major entry point into the European Union.
Only a handful of Red Cross workers were on hand at the station to give out food and provisions for the hundreds of babies and children there.
More help was on the way, said Kapic, including medical assistance and toilets.
"For now we have enough but more will be needed and is on the way," he said.
"It is very hard to say if this will become the next transit camp. It is down to the Croatian government how it deals with this," said Kapic.
He also said that the migrants were now coming straight to the train station without going to the police station for registration, with police overwhelmed by the numbers.
It was unclear where the migrants would go from Croatia, which borders to the north and north-east Schengen Zone members Slovenia and Hungary.
"I tried to take the train to Bulgaria but the line was closed so we had to come here," Hasan Shekh-Hasan, 25, a law student from Idlib in Syria, said at the station.
Having paid €115 for a taxi from the Serbian capital Belgrade to the border, he said he hoped Slovenia was open so he can keep going to his brother in Sweden.
"It is very hard here. There are so many people. We don't know what is happening," he said.