A group of migrants walk on the Serbian side of the border near Sid, Croatia September 16, 2015. [Photo/Agencies]

SID, Serbia - Barred from Hungary, migrants walked through cornfields into the European Union via Serbia's western border with Croatia on Wednesday, opening a new front in the continent's migration crisis.

Hundreds of migrants, some of whom identified themselves as Iraqis, treked through fields near the official Sid border crossing between Serbia and its fellow former Yugoslav republic, since 2013 a member of the EU.

They arrived by bus from the southern Serbian town of Presevo, rerouted late on Tuesday to the Croatian border after Hungary sealed its own frontier with Serbia to the wave of migrants streaming north across the Balkan peninsula en route to western Europe.

Serbian media reported that at least 10 migrant buses had left Presevo overnight bound for Sid. A Reuters television crew saw three arrive, one a double-decker that offloaded its passengers within a few hundred metres of the border.

Croatian police confirmed they had crossed, saying an unspecified number had been rounded up and would be registered and transported to reception centres near the capital, Zagreb.

From there, it is just 30 km to Slovenian border and Europe's Schengen zone of border-free travel. Many will hope to cross Slovenia into Austria and on to Germany and other prosperous countries of western and northern Europe.

Croatian Prime Minister Zoran Milanovic said they would be allowed passage through Croatia.

"Croatia is entirely ready to receive or direct those people where they want to go, which is obviously Germany or Scandinavian countries," Milanovic told parliament.

"They will be able to pass through Croatia and we will help, we're getting ready for that possibility," he said.

Croatia has space in reception centres for around 3,000 migrants, fewer than the number crossing borders daily up and down the Balkans, having landed in Greece by boat and dinghy from Turkey.

Some buses from Belgrade continued to carry migrants north towards the Hungarian border, now effectively sealed by a 3.5 metre-high fence and new rules that saw asylum seekers rejected in quick succession on Tuesday.