IDOMENI/NICOSIA: Macedonia trucked about 1,500 migrants and refugees back to Greece after they forced their way across the border on Monday, as European nations continued to pass the buck in a migration crisis that risks tearing the European Union apart.

The police action was part of a drive by Western Balkans states to shut down a migration route from Greece to Germany. Nearly a million people fleeing war and poverty in the Middle East and beyond used that route over the last year, forming biggest influx of refugees since World War Two.

But EU efforts to conclude a deal with Turkey to halt the human tide in return for political and economic rewards hit a setback on Tuesday. Cyprus, an EU member, vowed to block efforts to speed up Ankara's EU accession talks unless Turkey meets its obligations to recognise its nationhood.

European Council President Donald Tusk, who will chair a summit of EU leaders on Thursday and one with Turkey on Friday, flew on to Ankara to discuss the pact after talks with Cypriot President Nicos Anastasiades.

"Today we established a catalogue of issues that we need to address together if we are to reach an agreement by Friday," Tusk said after the talks in Ankara, adding that convincing all 28 EU states to sign on to the agreement was "not an easy task".

Tusk has acknowledged that the tentative deal put together last week by German Chancellor Angela Merkel and Dutch Prime Minister Mark Rutte with Turkish Prime Minister Ahmet Davutoglu raised legal problems and needed to be "rebalanced".

Davutoglu said the aim was to reduce illegal migration and make passage to Europe safe.

The European Commission meanwhile postponed proposals to reform the bloc's asylum system, which puts the onus on the state where migrants first arrive, in an attempt to avoid further controversy before the Turkey deal is done.

Some 43,000 migrants are bottled up in Greece, overstraining the economically shattered country's capacity to cope, and more continue to cross the Aegean daily from Turkey despite new NATO sea patrols.

On Monday, an estimated 1,500 people marched out of a squalid transit camp near the northern Greek town of Idomeni, hiked for hours along muddy paths and forded a rain-swollen river to get around the border fence.

Most were picked up by Macedonian security forces, put into trucks and driven back over the border into Greece late Monday or overnight, a Macedonian police official said.

"It's a long way from the camp to the mountains. It took me six hours of walking," said 60-year-old Mohammad Kattan, who slept rough in the mountains and trekked back on foot. "At my age it was very difficult. I would walk and rest often.

"My hope was to get to Macedonia, and get my papers stamped so that I could continue on to another country, to Serbia."

Another man forced back to Greece said the security forces with harsh with the group they had rounded up.

Greek authorities said there had been no official contact from Macedonia, so they could not confirm the return. Ties between the two neighbours are fraught because of Greece's long-standing refusal to recognise Macedonia's name, which is the same as that of a northern Greek province.

CONDITIONS DETERIORATING

A second group of about 600 migrants was prevented from crossing into Macedonia and many of them spent the night camping in the Greek mountains, according to a Reuters photographer.

At least 12,000 people, including thousands of children, have been stranded in the Idomeni camp, where sanitary conditions have deteriorated after days of heavy rain.

Scuffles have broken out in recent days as destitute people scrambled for food and firewood. Many have been sleeping in the open. Concern about the spread of infection grew after one person was diagnosed with Hepatitis A.

Greek Prime Minister Alexis Tsipras said on Tuesday there was "no chance" that border shutdowns throughout the Balkans would be lifted and urged refugees to move to reception centres set up by the state.

Jan van't Land, an official with medical charity Medecins Sans Frontieres at Idomeni, said around 400 migrants had returned to the camp.

"There are still many hundreds of people on both the Greek and the Macedonian side of the border," he told Reuters.

Greek officials say leaflets that circulated at the Idomeni camp before Monday's march showed it was a planned breakout.

"We are in possession of leaflets that show this was an organised incident, a very dangerous one, endangering people's lives," government spokesman George Kyritsis told reporters.

Babar Baloch, regional spokesman for U.N. refugee agency UNHCR who is at Idomeni, said the migrants' breakout and return "hasn't solved anything".

"It just increased sufferings of refugees. It started raining again. The sense of support for refugees in the region is missing," he said.

Turkey wants its citizens to have visa-free access to Europe by June and to open new "chapters" of its stalled negotiations to join the EU. In return, it will take back all migrants and refugees who cross to Greece or are fished out of its territorial waters.

PUSHING TURKEY DEAL

UN and EU officials doubt the legality of any blanket returns, and the UN human rights chief on Tuesday warned the EU risked compromising its human rights credentials with the Turkey deal.

Several EU countries, including France, also have misgivings about the more relaxed visa rules for Turkey, saying Ankara must first meet 72 criteria. But as the EU pushes to seal the deal, an EU official told Reuters on Tuesday Ankara would only be asked to meet a "critical mass" of them.

The European Parliament's foreign affairs committee on Tuesday called to keep the migration deal and Ankara's EU membership talks separate, citing concern with human rights in Turkey. And Cyprus is demanding that Turkey open its ports and airports to Cypriot traffic and recognise the island.

"I conveyed to President Tusk our position that the Republic of Cyprus does not intend to consent to the opening of any chapters if Turkey does not fulfil its obligations as described in the negotiating framework," Anastasiades told reporters after meeting with Tusk in Nicosia.

Dutch Foreign Minister Bert Koenders, whose country holds the EU's six-month rotating presidency, said the aim was to find an "intelligent synchronisation" between the diplomatic process to re-unify Cyprus and the EU-Turkey agreement.