Migrants disembark from a small boat as they arrive from the Turkish coast on the north-eastern Greek island of Lesbos (AP)

At least nine people, including four children, have died in the eastern Aegean Sea after a plastic boat carrying refugees or economic migrants overturned near the island of Kos, Greek authorities said.

The coast guard said it had rescued seven people and located nine bodies, two of which were still trapped inside the overturned vessel.

Crews were searching for between two and four more people listed as missing.

The dead were four children, four women and one man.

It was not immediately clear how the boat overturned, or what the passengers' nationality was.

Europe faces a massive refugee crisis. More than 600,000 people have made their way through Greece so far this year, fleeing war and poverty at home. The vast majority do not stay in the country, heading north through the Balkans to more prosperous European Union countries.

More than 500 people have died or gone missing since the beginning of the year.

PA Media