Turkey's coast guard says it has rescued 31 migrants after their boat sank off the Turkish coast and is searching for an estimated nine others who are unaccounted-for.

The coast guard said the boat went down in the Aegean Sea off the coast of Bodrum early on Monday. The region is close to the Greek island of Kos.

Two coast guard boats, a helicopter and a team of divers were involved in the search-and-rescue operation, the coast guard said.

It was not immediately clear why the boat sank.

Although the number of people heading to the Greek islands from the Turkish coast has decreased since the height of the refugee crisis in 2015, hundreds continue to make the crossing each week.