ATHENS (Reuters) – Thousands of migrants were stranded in northern Greece on Monday after neighboring Macedonia demanded additional identification from people seeking to cross the border and head to Western Europe, witnesses said.

European leaders are concerned that migrants passing through austerity-hit Greece to more prosperous countries could end up stranded if Greece’s northern neighbors tighten border controls.

Greek officials say the flow of people across the border slowed after Macedonia demanded additional identification from people seeking passage.

About 5,000 people massed at two locations in northern Greece, close to the border with Macedonia, while aid groups urged another 4,000, who arrived on the Greek mainland from outlying islands, not to head to north for fear of creating a bottleneck.

“Our biggest fear is that the 4,000 migrants who are in Athens head up here and the place will become overcrowded,” said Antonis Rigas, a coordinator of the medical relief charity Medicins Sans Frontieres (Doctors Without Borders).

Balkan states straddling the migrant route to western and northern Europe have begun denying passage to individuals not coming from the conflict regions of Syria and Iraq.

One migrant in his mid-30s, who said he was from the Syrian town of Aleppo, said Macedonian police did not let him cross the border because he did not have a passport.

“I lost everything in the war, I have no documents,” he said, declining to give his name. He said he had obtained Greek registration papers at the island of Lesbos.

Macedonia has erected a metal fence topped with razor wire at the main crossing point for migrants along its southern border with Greece.

Greek migration minister Yannis Mouzalas criticized his neighbors for shirking their responsibilities amid the crisis.

“Not only have Visegrad countries not taken in one refugee, they didn’t even send a blanket or a tent,” he told the TV channel of Greece’s parliament, referring to the Czech Republic, Poland, Hungary and Slovakia.

They had not sent a ‘single policeman’ to reinforce the EU border agency Frontex either, he said.

Austria has invited Balkan states to a meeting on migration in Vienna on Feb. 24, a day before EU interior ministers are due to meet on the migrant crisis.

Vienna has angered other EU members by imposing a cap on asylum claims, limiting the number of migrants permitted into its territory to 3,200 per day, and introducing a daily cap of 80 asylum claims.

Its interior minister has said Austria could impose even stricter controls, a move that could trigger other countries north of Greece to do the same.

(Additional reporting by Fedja Grulovic; writing by Michele Kambas; editing by Katharine Houreld)