Almost 30 Syrian refugees who had their initial bids for asylum in Greece rejected have been returned to Turkey.

Greek officials said the 17 men, four women and six children were flown from the island of Lesbos to Adana on Thursday.

Under a deal struck last March between the European Union and Turkey, migrants reaching the Greek islands from Turkey are returned unless they can prove they merit asylum in Greece.

More than 800 people have now been returned under the agreement, while about 23,000 new arrivals have reached the Greek islands.

Meanwhile, the livelihoods of two Aegean island fishermen credited with saving hundreds of lives during the height of the migration crisis are being threatened by the winter freeze gripping Greece.

Heavy snow on Lesbos this week sunk the boats of the two men, who were nominated for the Nobel Peace Prize for their efforts to save refugees and other migrants from drowning.

Thanassis Marmarinos said losing his boat in the small harbour of Skala Sikaminias was a major blow, and all his navigational equipment was also ruined.

Island mayor Spyridon Galinos has pledged to help compensate them with prize money from Sweden's Olof Palme Memorial Fund that recognised the humanitarian acts of Lesbos residents.

It comes as the leader of the European Union's presidency warned record numbers of migrants will try to cross the Mediterranean this spring.

Maltese prime minister Joseph Muscat said "we will have a crisis", as he urged the EU to work with Libya now to stem the flow.

He said Europe can choose between "trying to do something now, or meeting urgently in April, May... and try to do a deal then".