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Thousands of refugees and migrants walking through Croatia towards western Europe face a “real danger” from minefields laid during the 1990s Balkans War, volunteers warned Thursday.

It poses a new hazard for the wave of families fleeing conflict in Syria, Iraq and Afghanistan.

The Croatian Mine Action Center reports that the country has more than 50,000 active landmines spread across almost a 200-square-mile zone described by the International Red Cross as "among the most mine-affected in Europe."

A map produced by the organization shows a heavy concentration of mines near the Serbian border with Croatia.

They were laid during the four-year conflict that followed the collapse of Communist Yugoslavia in 1991.

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Hungary on Monday closed its southern border with Serbia and police used water cannons, tear gas and batons to beat back migrants, arresting anyone trying to enter illegally.

With their main route towards Germany closed, many migrants have opted to travel through Croatia.

Police in Croatia said Thursday that 6,200 people had entered the country since the first groups started arriving early Wednesday.

“While they have been very welcoming many migrants try to avoid the police after their experience in Hungary,” said Zsuzsanna Zsohar of Migration Aid, a civil initiative of volunteers in Hungary helping refugees.

“So there is a very real danger that they won’t use the roads, but will instead try to cross the border through fields. The trouble is they may be minefields,” she added.

Minefields have signs in Croatian warning of their dangers, but Zsohar said it was still possible some people might inadvertently walk into the fields.

She said Migration Aid had been issuing warnings on Facebook and via a mobile phone apps, as well as putting up signs along the routes in Serbia to warn them. Other groups have also posed maps on social media.

“We are trying to provide them with food and water, but most importantly information,” she said.

Croatia's government and mine clearance groups hope the country will be free of the explosive devices by 2019.

“It would be horrible if people leaving war-torn countries died in a minefield in Croatia,” said Zsohar. “So we are trying to help them make their journey safely.