The growing tensions at the heart of the EU's flagship Schengen area ramped up the pressure on interior ministers holding emergency talks in Brussels on the unprecedented flood of people fleeing Syria and other war zones.
A day before Hungary has vowed to begin arresting illegal migrants, police fenced off a gap in the razor-wire barrier with non-EU Serbia that hardline Prime Minister Viktor Orban's government is racing to complete, leaving dozens of migrants stranded.
Despite the escalating crisis and a shift in public opinion after heart-wrenching pictures of a drowned Syrian migrant child -- one of more than 2,700 people to have died crossing the Mediterranean this year -- EU ministers remained divided on a solution.
"If we do not take decisions then chaos is the consequence... There will be a domino effect and we can forget Schengen," said Luxembourg minister Jean Asselborn, who chaired the EU talks in Brussels.
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At Hungary's Roszke border crossing, several dozen migrants including many children, some in pushchairs, were stuck on the Serbian side of the border, with several women crying, after police shut the border, AFP journalists saw.
They were directed to the official crossing around two kilometres (1.6 miles away) where there was a heavy police presence with helicopters overhead, a sign of the tough welcome many migrants say they face in Hungary.
Hungary is on the frontline of Europe's migrant crisis, with almost 200,000 people travelling up from Greece through the western Balkans and entering the country this year, most of them seeking to go to northern Europe.
But Hungary's neighbours are now feeling the strain, with Germany shocking the EU yesterday when it admitted that Europe's biggest migration crisis since World War II meant it had to reinstate border controls eliminated under Schengen in the late 1990s.