"We forced them, by sending people up there. And we'll keep doing it," a defiant Zoran Milanovic told reporters.

Croatia has transported hundreds of refugees on buses and trains back to Hungary after officials said they could not cope with the 20,700 who have arrived in just a few days.

They had entered Croatia looking for an alternative route to western Europe after neighbouring Hungary closed its border with Serbia and finished putting up a razor wire fence.

Now, after being taken from Croatia to Hungary some of the refugees are being transported by Hungarian buses to registration centres in the north of the country, near the border with Austria.

Sky's Mark Stone, who is in Beremend on the Hungary-Croatia border, said: "From the migrants' point of view this is good news. A bottleneck has been released.

"Hungary is allowing them across. Effectively it has opened up a safe corridor for them to get to Austria.

"If they leave those (registration) camps and walk across into Austria there is nothing Hungary can do about it."

Mr Milanovic was speaking after Hungary accused Croatia of "a major violation of international law" after it tried to take 1,000 refugees by train across its border.

Budapest described it as a "major, major incident" and alleged 40 police officers on board had to be disarmed and sent back.

Hungarian government spokesman Zoltan Kovacs said it showed "the Croatian system for handling migrants and refugees has collapsed, basically in one day."

"What we see today is a complete failure of the Croatian state to handle migration issues," he added.

"What is more, we see intentional participation in human smuggling, taking these refugees to the Hungarian border.

"The Croatians have started on purpose and in a planned way to transport the illegal migrants to us, letting everyone down.

"They have forced the illegal migrants on us Hungarians, they transported them here breaking all European rules, not abiding by any protocol with state aid.

"They have let down not only Hungary but the whole European Union."

According to local media, Hungary has now called up military reservists to deal with the "mass migration situation."

As well as building another razor-wire fence on its border with Croatia, it has helicopters scouring the countryside and soldiers keeping watch in armoured vehicles.

Crawford says busloads of refugees are now arriving at the Hungary-Austria border at Hegyeshalom.

Austrian police say around 6,700 have been allowed into the country since midnight.

In other developments, Slovenian riot police used pepper spray at the Harmica crossing overnight as they clashed with about 500 refugeestrying to cross from Croatia.

They say they were forced to act after sticks and bottles were thrown.

Limited numbers are being let into the country - mainly families with children - and authorities say they will take them to refugee centres.

Slovenian police said on Saturday more than 1,500 refugees have entered the country while 600 more were waiting at the border crossings with Croatia to get in.

One third of the registered refugees said they were from Syria and another third from Afghanistan. The rest said they were from Iraq and various other nations in the Middle East, Asia or Africa.

Most of the refugeesare still adamant on claiming asylum in Germany and Scandinavia.

Hundreds of people are also camped by a motorway at Edirne, near Turkey's border with Greece and Bulgaria, hopeful of soon setting foot on European Union soil.

EU leaders are holding an urgent meeting on Wednesday to try to find a common strategy on the crisis and ease growing political tensions.