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ZVORNIK, Bosnia (Reuters) - Bosnian police stopped about 50 migrants from Afghanistan, Iran and Pakistan trying to cross from Serbia in a cargo train overnight on Friday as authorities step up patrols of the border.

Bosnia had not been on the well-trodden migrant route through the Balkans to European Union nations until earlier this year when arrivals jumped. About 11,000 people from Asia and North Africa have been registered as arriving so far this year.

Still recovering from a devastating war in the 1990s, Bosnia was bypassed in 2015 and 2016 when more than a million migrants and refugees passed through the Balkans. But as borders have closed elsewhere, Bosnia has become a new transit route.

“Migrants are crossing the River Drina (the border between Serbia and Bosnia). They pay between 250 and 300 euros to smugglers to take them over the Drina,” said a police source who asked not to be named.

The cargo train carrying coal was stopped near the Bosnian border town of Zvornik on the banks of the River Drina. As the train was brought to a halt, some of the migrants onboard jumped out and started running, some back to Serbia, said a Reuters photographer who witnessed the operation.

A group remained on the train and were then turned back to Serbia, the photographer and police said. Local police did not give further details.

Migrants and asylum seekers, most of them from Afghanistan, Iran, Syria and Pakistan, reach Serbia via Albania and Montenegro, according to Bosnian authorities.

Some go to beaches on the Serbian side pretending to be tourists and then swim to the Bosnian side, the police source said. They then walk west to try to cross into the EU member Croatia.

But with Croatian police increasing border patrols and turning back migrants, around 5,000 remain stranded around in Bosnia.