EU efforts to conclude a deal with Turkey to halt the human tide in return for political and economic rewards hit a setback as Macedonia trucked about 1500 migrants and refugees back to Greece after they forced their way across the border.
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.
Cyprus, an EU member, vowed on Tuesday 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.
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.
About 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 1500 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 on Monday or overnight, a Macedonian police official said.
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.
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.
Babar Baloch, regional spokesman for UN refugee agency UNHCR who is at Idomeni, said the migrants' breakout and return "hasn't solved anything".
