Macedonia moved to cut off the flow of migrants pouring over its southern border with Greece on Thursday, deploying riot police in armored vehicles and calling out the army under a state of emergency.
Authorities said official border crossings remained open, but that they would "reduce illegal border entry to a minimum".
A Reuters reporter near the border town of Gevgelija said a column of riot police armed with tear gas and armored vehicles had shut off passage for several thousand people now stranded in no-man's land.
"No more Macedonia," one officer said in English to a Syrian man requesting passage.
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The flow into Gevgelija, which has hit 1,500 to 2,000 a day, suddenly stopped. The shutdown came after days of desperate scenes at the local railway station as thousands of people pressed to board trains to Serbia, young children being passed through open carriage windows.
Macedonia acted as a Greek car ferry docked in Athens carrying 2,400 Syrian refugees from the island of Kos, just some of the 50,000 Middle Eastern, African and Asian migrants and refugees who arrived in Greece in July alone. Many will take buses north, heading for Macedonia, then Serbia and Europe's borderless Schengen zone in Hungary.
"We cannot hermetically close the borders," Interior Ministry spokesman Ivo Kotevski told Reuters. But "we will try to reduce illegal border entry to a minimum."
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Kotevski said there was no coordination between Greek and Macedonian police. The two countries have has an uneasy relationship, rooted in a dispute over Macedonia's name, since it declared independence from Yugoslavia in 1991, a row that has blocked Skopje's integration with NATO and the European Union.
Kotevski said a state of emergency had been declared on the northern and southern borders and that soldiers would be brought in to help address a crisis that is straining the capacity of cash-strapped Macedonia and Serbia.
To the north, Hungary is racing to complete a fence along its 175-km border with Serbia to keep them out, threatening to create a bottleneck of tens of thousands.
Macedonia appealed on Wednesday for neighboring countries to send train carriages to address the demand. But the United Nations refugee agency urged the government to do more, saying it should allocate a site to accommodate the migrants and refugees.
"Depending on how Greece uses ships to de-congest the islands, that will also temporarily increase the arrivals here,” Alexandra Krause, senior protection officer at the UNHCR in the Macedonian capital, Skopje.
"The (Macedonian) government needs to provide an appropriate site to be able to shelter the arrivals properly and to ensure sufficient assistance,” Krause told Reuters.
The only site currently being used is at the local police station, where Krause said the UNHCR had constructed some shelter with capacity for just 165 people. Krause said the Red Cross had access to the migrants and refugees in the border area but warned of harsher weather approaching.
(Additional reporting by Jaksa Scekic, Aleksandar Vasovic and Matt Robinson in Belgrade; Writing by Matt Robinson; Editing by Larry King)