At least 11 migrants, including five children, drowned and about 10 were missing after their boat sank off the Greek island of Farmakonisi, Greek maritime police said on Wednesday.

Twenty-six people were saved and a search was underway in the southeastern Aegean Sea for other survivors from the capsized wooden boat.

A Greek army helicopter was pressed into service with a vessel from the European Union's border agency Frontex to aid the search, police said.

The dead also included two women and four men.

Witnesses said the boat, which set off from the Turkish coast, sank despite a calm sea and light winds.

The latest tragedy saw the number of children drowned at sea rise to 11 in two days.

On Tuesday, at least six children died when a dinghy carrying Afghan migrants sank in Turkish waters while on its way to Greece.

The Turkish coast guard recovered the bodies of the children, including a baby, and was still looking for two other migrants reported missing, the official Anatolia news agency said.

The inflatable vessel sank in strong winds and high waves near the town of Cesme in the western province of Izmir, the agency said. It was apparently heading for the Greek island of Chios.

More than 886,000 migrants have arrived in Europe by sea so far this year, according to the latest United Nations figures.

More than 650,000 of them - mainly refugees from war-torn Syria - have undertaken sea journeys from Turkey to Greece to enter the European Union.