Idomeni - Greek police removed hundreds of protesting migrants blocking a railroad border crossing, as concern grows that people pouring in from Turkey and running into a closed border may overwhelm the country's sheltering capacity.

Around 900 people had been blocking the track since Monday in protest at Macedonia's policy of allowing only Syrian and Iraqi refugees to cross its borders while stopping all others.

Police transported the mostly Afghan protestersinland, away from the border.

But Greece may quicklyrun out ofshelters if Macedonia continues stoppingAfghans from entering,an international aid organisation warned on Tuesday.

"IfMacedonia does not re-open it borderto Afghans soon, Greece's sheltering capacity will be fullwithin eight days," aDoctors Without Borders spokesperson, Gemma Gillie, told dpa at Idomeni.

The Greek Defence Ministry meanwhile held an emergency meeting to discuss openingold army barracks to stranded migrants, a source from the government tolddpa in Athens.

Thousands of migrants have amassed in theGreek village of Idomeni on the border with Macedonia since Skopje on Sunday stopped allowing Afghans to pass into the country along with Syrian and Iraqi refugees.

The Afghanshad previouslybeen allowed passage along the Balkan migration route from Greece, through Macedonia, Serbia, Croatia and Slovenia to Austria.

Balkan countries in November began turning back all others -mostly from Pakistan, Bangladeshand Africa -regarding them as economic migrants without a chance of receiving asylum in EU countries.

After Austria capped the number of asylum seekers it would take to 80 daily, allowing a further 3 200 to pass on to Germany, other Balkan route countries also took steps to limit the influx of migrants.

The Slovenian Parliament late on Monday - in a rare display of bipartisan unity -authorised with an overwhelming majority the army to police the borderwith Croatia.

The measure is intended to suppress trespassing and people-smuggling, which has been on the rise in the Balkans since the transit countries began pushing the door shut in recent months.

Filtering war-zone refugees from thousands of economic migrants places Greeceunder huge pressure.

Unable to stop people from arriving at its Aegean islands from the nearby Turkish coast, Greeceworriesthat it will end upsheltering tens of thousands of stranded migrants.

Only on Tuesday morning, another 1 250migrants disembarked in Athens from ferries that brought them from the islands, the Greek Coast Guard said.

The authorities are trying to channel the migrants to a collecting centre in a sports arena near the port of Piraeus, but most attempt to continue the journey north on their own, reports say.

Nearly 1 millionpeople traversed the Balkan migration route in 2015, intending to seekasylum in Germany and other wealthy countries.The bulk arrived in the latter half of that year and the surge continues in 2016.

According to International Organisation for Migration figures released Tuesday, more than 100 000 refugees and migrants have already crossed the Mediterranean - nearly all via the Aegean fromTurkey to Greece - sincethe start of 2016.