Greece is trying to figure out how it will accommodate record numbers of migrants stranded there should countries further along the Balkan migration route block access for refugees.
Greece is counting on "unilateral moves" from Macedonia, the minister in charge of migrants, Ioannis Mouzalas, told Greek TV channel Mega Wednesday. A day earlier, he said that closing the Balkan borders would strand tens of thousands of migrants in the European Union's south-eastern-most country.
Around 1 million people, including many refugees from war zones in the Middle East, passed along the Balkan route in 2015, most of them in the latter half of the year.
They mostly sailed from Turkey to Greek islands in the Aegean Sea, then crossed through Balkan countries such as Macedonia, Serbia, Hungary, Croatia and Slovenia, intending to reach Austria and then Germany and other wealthy northern European countries.
Last month, Austria set off a wave of responses from countries upstream on the Balkan route, after saying it will limit the influx of asylum seekers to 37,500 this year and a total of 127,500 until mid-2019.
Slovenia, Croatia, Serbia and Macedonia - which are between Austria and Greece on the Balkan route - immediately responded with a filter allowing only Syrians, Afghans and Iraqis to pass to the north.
Many of the countries along the route are primarily equipped for short-term migrant stays, limiting their ability to take on refugees if the path closes further downstream.
Macedonia on Tuesday began reinforcing and expanding a fence along its southern border. On Wednesday, Macedonian authorities began setting up a camp for migrants turned back by countries further along the route which are also narrowing the gates to new entries.
Serbia's capacity to shelter people for a long period of time would be exhausted within three days at present arrival rates, the UN refugee agency UNHCR warned.
The 17 existing centres for asylum seekers and migrants in Serbia are equipped only for emergency shelter, not residence, UNHCR officials told the Belgrade daily Danas.
If other countries on the route close their gates, Serbia will immediately do the same, Foreign Minister Ivica Dacic said after meeting with Austrian Foreign Minister Sebastian Kurz on Tuesday.
The European Union has warned countries on the Balkan route not to take unilateral action. Its executive, the European Commission, said Wednesday that tightening border controls will undermine actions that have been taken to alleviate the migration pressure.
With the EU struggling to get a handle on the migration crisis, NATO defence ministers were due to consider Wednesday a possible role for their alliance, following a proposal by Turkey.
"We will look very seriously into the request," NATO Secretary General Jens Stoltenberg said.
German Defence Minister Ursula von der Leyen gave her support to Ankara's request for NATO maritime surveillance between Turkey and Greece, and said Berlin would contribute to such a mission.
But EU Migration Commissioner Dimitris Avramopoulos downplayed the idea, pointing to Italy and Greece's "spectacular" naval fleets.
"NATO defines its role and its existence by ... the element of threat," Avramopoulos told journalists in Brussels. "Do you see any threat there? We need to cope with human beings."
The Italian Coastguard - which runs patrols in Greek waters as part of an EU rescue mission - said Wednesday that it rescued 18 people overnight, including two suspected Turkish human smugglers who were handed over to Greek police. Other passengers claimed to be Syrians.