Brussels (dpa) - Greece might be sliding into a humanitarian crisis, European Union officials warned Tuesday as thousands of migrants became stranded after Macedonia sealed its border, while thousands more arrived from Turkey.

"We are concerned about the developments along the Balkan route and the humanitarian crisis that might unfold in certain countries, especially in Greece," EU Migration Commissioner Dimitris Avramopoulos and Dutch Migration Minister Klaas Dijkhoff said in a joint statement.

Avramopoulos and Dijkhoff urged EU member states on the path of the migrants to cooperate "in a joint spirit of solidarity and responsibility."

The countries along the Balkan migrant route include EU members Greece, Croatia, Slovenia and Austria as well as neighbouring countries Turkey, Macedonia and Serbia.

But Greece, which already complained of lacking solidarity in Europe, lashed out with a threat that it may use a veto to block EU membership bids over migration policies.

"We are not excluding it [the possibility of veto]," Greek government spokeswoman Olga Gerovasili said.

She did not name anyone. But Athens has been angered by Turkey, which is the source of refugees and migrants arriving on the Greek Aegean islands, while Macedonia and Serbia have both stopped taking in many migrants, leaving them stranded in Greece.

Turkey on Tuesday signaled reluctance to take back migrants plucked from the Aegean by ships deployed within a NATO agreement, with Deputy Prime Minister Numan Kutulmus saying that this particular point was still being negotiated.

On the northern side, Serbia and Macedonia, as well as EU members Croatia and Slovenia, are worried that thousands of people may become stuck on their soil if Germany or Austria decide to shut their gates.

Macedonia on Sunday began stopping people from Afghanistan and now only allows Syrians and Iraqis to continue north. Afghans had been waved through in recent months, unlike all others regarded as economic migrants.

The latest in the series of filters wreaked havoc in Greece, with up to 6,000 people stuck at Idomeni, a village on the border with Macedonia, as thousands were still arriving on the islands and at Athens' port Piraeus.

The crowding was so severe that Greece may within days run out of shelters, aid workers at Idomeni warned.

"If Macedonia does not re-open its border to Afghans soon, Greece's sheltering capacity will be full within eight days," a Doctors Without Borders spokeswoman, Gemma Gillie, told dpa.

The UN refugee agency UNHCR also criticized the migration policy on the Balkan route, saying that it was "creating chaos at several border points" and complicating the situation in Greece.

Austria and several Balkan countries are "putting particular pressure on Greece as it struggles to deal with larger numbers of people in need of accommodation and services," UNHCR said.

Greek police on Monday cleared a crowd of around 1,000 people, mostly Afghans, blocking a railroad track at the border crossing near Idomeni and transported them inland.

The Greek Defense Ministry meanwhile held an emergency meeting to discuss opening old army barracks to stranded migrants, a government source told dpa in Athens.

Only on Tuesday, another 1,500 migrants arrived in Athens from the islands, the Greek Coast Guard said.

The authorities are trying to channel them 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.

According to earlier local reports, the authorities are not sending buses to transport migrants north to reduce pressure at the border.

Nearly 1 million people traversed the Balkan migration route in 2015 intending to seek asylum in Germany and other wealthy countries, and the surge continues in 2016.

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