Hungary has effectively stopped registering thousands of migrants crossing the border from Serbia and is transporting them straight to the Austrian frontier, the UN refugee agency said Monday.

EU member Hungary has become a frontline state in Europe's migrant crisis, with more than 180,000 people travelling up from Greece through the western Balkans and entering the country this year.

In addition to the new laws, Hungary is also building a controversial four-metre high (13-feet) fence all along its 175-kilometre (110-mile) border with Serbia.

On Sunday, however, Germany reimposed border controls and trains from Austria were suspended -- although they resumed Monday morning -- leaving thousands arriving from Hungary effectively stranded.

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