According to the Greek coast guard, three migrants drowned on Tuesday while attempting to cross from Turkey to Greece after a boat carrying around 27 passengers sank.

Greece is one of the main entry points into the European Union for asylum seekers arriving in flimsy, overloaded rubber boats from Turkey, but the number of people arriving has dropped dramatically in recent years, and tragic shipwrecks in Greek waters have become uncommon.

Authorities said 21 people were rescued and three more were believed to be missing. Ten vessels and two helicopters were assisting in the rescue operation.

Nearly 1 million people, mainly Syrian refugees, arrived in the EU in 2015 after crossing to Greek islands close to Turkey.

About 6,500 asylum-seekers have arrived in Greece this year, most through its northerneastern land border with Turkey, according to data by the United Nations refugee agency UNHCR.