Migrants walked through cornfields into the European Union through Serbia's western border with Croatia on Wednesday, opening a new front in the continent's migration crisis after Hungary shut the main overland route.
Croatia said it was urgently sending demining experts to the border area to identify minefields left on the frontier from the Balkan wars of the 1990s, the last time hundreds of thousands of displaced people marched across Europe.
Hungary's decision to shut the EU's external border with Serbia this week was the most forceful attempt yet by a European country to close off the unprecedented flow of refugees and economic migrants overwhelming the bloc.
The route through Hungary has been the main one used by migrants who arrive first by dinghy in Greece and then trek across the Balkan peninsula to reach the EU's frontier-free Schengen zone, most eventually bound for Germany.
With that route closed, thousands of migrants remain in the Balkans seeking other paths north and west, possibly through Croatia and Romania, both of which are in the EU but not in Schengen.
Reuters reporters saw hundreds of people, some of whom identified themselves as Iraqis, trek through fields near the official Sid border crossing between Serbia and Croatia, a fellow former Yugoslav republic which joined the EU in 2013.
They arrived by bus from the southern Serbian town of Presevo, rerouted late on Tuesday to the Croatian border after the Hungarian border shut.
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.
Hungary has thrown up a 3.5 metre (10 foot) high fence along the length of its border with Serbia. Engineers and soldiers were marking out a path on Wednesday to extend the fence along the border with Romania, a plan that has angered Bucharest.
The biggest flow of immigrants into Western Europe since World War Two has sown discord across the continent, fuelling the rise of far right political parties and jeopardising the 20-year-old achievement of Schengen's border-free travel.
Hungary says it is simply enforcing EU rules by sealing the Schengen zone's external border. It says Serbia is a safe country, so asylum seekers who reach the frontier there can be automatically turned back in a process that should take hours.
The United Nations says Serbia lacks the capacity to receive refugees halted at the gates of Europe. Critics at home and abroad say Hungarian Prime Minister Viktor Orban's rhetoric - he has said he is defending Europe's "Christian values" from the mainly Muslim migrants - has crossed the line into xenophobia.
The crisis has pitted countries that are comparatively open, led by Germany, against those, many in former Communist eastern Europe, who argue that the welcoming approach has made the problem worse by encouraging people to make dangerous voyages.
Hungary blames Germany for exacerbating the crisis by announcing in August it would suspend normal EU asylum rules and take in Syrian refugees regardless of where they enter the EU. Thousands have since been trekking across the bloc, mainly through Hungary and Austria, to reach Germany, clogging railway stations and forcing trains to be cancelled.
Record numbers rushed to cross Hungary in the days before the border was shut, with thousands now backed up in Austria trying to reach Germany.
Germany ordered the emergency reintroduction of border controls on Sunday to divert migrants away from Munich, the southern city that had been overwhelmed by tens of thousands arriving within days. Austria and Slovakia said Germany's move left them no choice but to impose similar controls.
An emergency meeting of EU ministers failed this week to reach agreement on a Berlin-backed plan to share out 160,000 refugees across the bloc. A German cabinet minister said on Tuesday the EU should consider financial penalties against countries that refuse to take their share, drawing angry responses from countries which oppose quotas, such as the Czech Republic and Slovakia.
Croatia said it would allow migrants who arrive Serbia to continue onwards. It has a border with Slovenia, which could provide the migrants with a new route into the Schengen zone.
"Croatia is entirely ready to receive or direct those people where they want to go, which is obviously Germany or Scandinavian countries," Prime Minister Zoran Milanovic said.
"They will be able to pass through Croatia and we will help, we're getting ready for that possibility."
There was little sign so far that the new difficulty crossing the Balkans was slowing the flow of migrants out of Turkey, which has been housing 2 million Syrian and Iraqi refugees.
Hundreds of migrants, mainly Syrians, spent a night out in the open near Turkey's land border with Greece after Turkish police blocked them reaching the frontier.
"I am young I am strong, if I can make it to Europe perhaps I can have a life. We have degrees, we have education, there's nothing for us here in Turkey," said 25-year old Saleh, an electronics engineer from the Syrian city of Aleppo.
Most reach Greece by sea across the Aegean, like 26-year-old Abeer, a Syrian refugee who arrived by boat before dawn. She was waiting with her two daughters outside a travel agency in Athens for five hours while her husband Ihab went to collect money wired in by his brother from Germany. When he returned, they bought bus tickets north to the border with Macedonia to trek across the Balkans.
"I never thought that one day I would find myself in such a situation," said Abeer, whose husband was a health ministry employee in Syria's city of Deir al-Zor.
"I am ashamed to expose myself in such a way. I feel like a beggar."
Read:Hungary seals border with Serbia, detains migrants