Stranded migrants scuffle with Greek police officers as they try to cross the Greek-Macedonian border, near the village of Idomeni, Greece December 2, 2015.

Macedonian police fired tear gas at hundreds of mostly Pakistani migrants who tried to storm into the Balkan country from Greece on Wednesday demanding passage to northern Europe.

About 1,500 Pakistanis, Moroccans and Iranians have been stuck in no-man’s land between Greece and Macedonia for weeks after non-EU Balkan states began filtering migrants and granting passage only to refugees fleeing Syria, Iraq and Afghanistan.

Protests have swelled among desperate migrants stranded for days in squalid tent camps on the border near the Greek town of Idomeni in temperatures barely above freezing.

Wednesday’s violence broke out after about 200 people were denied passage and began walking, for several kilometers (miles), alongside a newly erected border fence, seeking an alternative opening.

Macedonian police fired tear gas to push back the crowd and one officer fired warning shots in the air, a Reuters witness said.

At dusk, about 500 migrants blocked the crossing for refugees in protest shouting: “If we don’t cross, no one does!”. Police stood guard. Buses full of migrants and refugees who have landed in other cities in recent days kept arriving.

Tensions had already boiled over at the weekend after one migrant, believed to be Moroccan, was electrocuted and badly burned when he climbed on top of a rail wagon.

More than 800,000 people, most fleeing war, persecution and poverty in the Middle East, have arrived in Europe this year. The continent has struggled to cope with the unprecedented stream of people.

The decision to screen migrants based on nationality has drawn criticism from human rights groups that argue asylum requests should be treated on merit.

Greek Migration Minister Yannis Mouzalas said the government was trying to persuade the estimated 1,500 migrants stuck at the border to come to Athens and apply for asylum in Greece, saying there was accommodation available for them.

Some had told Greek officials they were willing to return to their home countries, but the Pakistani government was not responding to requests to readmit them and the International Organization for Migration had no money to fly them home.

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