Croatia will demand that Greece stop moving migrants from the Middle East on towards the rest of Europe, Interior Minister Ranko Ostojic said on Monday.
EU interior ministers will meet on Tuesday in an attempt to find a solution to Europe's biggest migrant crisis since World War Two, with almost half a million asylum seekers reaching its territory this year.
"The flow of migrants from Greece must be stopped. I will seek that at tomorrow's meeting of EU interior ministers," Ostojic told reporters at the Opatovac camp where migrants are being housed near the eastern town of Tovarnik.
"It is absolutely unacceptable to have Greece emptying its refugee camps and sending people towards Croatia via Macedonia and Serbia," Ostojic added.
Around 29,000 people, mostly from Syria, have arrived in Croatia from Serbia in the past week after trekking northwards through the Balkans from Greece en route to wealthier country in the west and north of the European Union. Croatia is a member of the EU but not part of its Schengen zone of borderless travel.
Greece has been the first point of entry to the EU for many migrants as it borders Turkey, to which millions have fled from wars in neighbouring Syria and Iraq, but says it cannot cope with the influx given its small size and severe financial woes.
An official of the U.N. refugee agency, UNHCR, told Reuters there were currently around 2,000 people inside the Opatovac camp, a fenced former industrial plant where around 150 olive-coloured, military-style tents have been set up.
"On the average there are around 100 people entering (Croatia) per hour," UNHCR spokesman Babar Baloch said.
"Of course, inflow and outflow must be balanced or (the camp ) will become overcrowded... This of course depends on the number of people arriving from Serbia," he said.
A Reuters reporter saw hundreds of new migrants arriving in Opatovac on Monday morning, after having been brought from the Serbian border by Croatian police. Local Red Cross workers gave them food and water as they awaited registration by authorities.
"This is good...we will rest here and will go to Finland from here," said Osama, a refugee from the Iraqi city of Mosul. He had travelled with three cousins for more than four weeks. "We have no money, that's why we travel slow, we walked a lot."
Asked why he was headed to Finland, Osama replied: "It is far away from Iraq and the Islamic State (insurgents); no war there". Islamic State captured Mosul in June 2014.