ATHENS/OSLO - Thousands of migrants were stranded in northern Greece on Monday after neighbouring 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 neighbours 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 criticised his neighbours 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.

Meanwhile, volunteer street patrols calling themselves the Soldiers of Odin and claiming to protect locals from asylum seekers walked the streets of several Norwegian towns this weekend, as the group founded in Finland branches out across northern Europe.

Dozens of men, some wearing black bomber jackets emblazoned with the group's Viking helmet logo, gathered on Saturday night in the streets of Stavanger, Drammen and Kristiansand, the group said Monday.

"We want the streets to be safe, we want to get rid of the delinquency we see in Norway today which the police are unable to address," the group's Norwegian spokesman Ronny Alte told AFP. "Drugs are being sold, girls are being touched, there are assaults and violence."

The group is based on the Finnish Soldiers of Odin, which has links to neo-Nazis and was founded last year in response to a record number of migrants and refugees arriving in Europe, claiming that the influx has led to a rise in crime.

It now also has branches in Denmark and Sweden.

Alte, who was formerly a member of Islamophobic groupings such as the Norwegian Defence League and German-based PEGIDA, said the Norwegian branch of Soldiers of Odin "represents the entire political spectrum" and comes to the aid of everyone.

"But a large part of the crime we are focusing on is the result of the illegal immigration in Norway after Europe opened its borders," he said.

Like their Finnish counterparts, Norwegian police said they were "generally sceptical of groups like the Soldiers of Odin."

"The use of names (Odin is a god of war in Norse mythology) and symbols, and the fact that some of the members of the group have links to criminal circles, cement this scepticism," Atle Roll-Matthiesen, a high-ranking police official, told AFP.

"It's not acceptable that groups act or give the impression of being some kind of citizen self-defence group... Only the police are authorised to carry out policing duties," he added.

In Drammen, police on Saturday checked the identities of the patrol members and verified that they were unarmed.

In Kristiansand, the members were only authorised to hand out pastries and coffee. The Norwegian intelligence agency PST's annual report published on February 9 said the "threat from far-right circles was growing in Norway" as a result of the migrant crisis. The Nordic country of 5.2 million saw a record 31,000 migrant arrivals last year.