ISTANBUL (REUTERS) - Eighteen people, including six children, drowned on Thursday after a boat carrying migrants sank off Turkey's western coastal town of Dikili, Dogan news agency reported.

The Turkish coast guard rescued another 21 migrants from the sea including a one-year-old baby and is still searching for two more people from the wooden boat, which capsized due to rough seas and overloading, the agency said.

The coast guard was not immediately available for comment.

The number of refugees and migrants arriving by land and sea in the European Union has passed 1 million this year, and almost 3,700 died or went missing, the International Organisation for Migration said on Tuesday.

At least half a million refugees fleeing Syria's civil war have travelled this year through Turkey, then by sea to Greek islands, their first stop in the European Union (EU).

Despite the winter conditions and rough seas, the exodus has continued, albeit at a slower pace. Hundreds have died this year on this sea route.

Turkey struck a deal with the EU in November pledging to help stem the flow of migrants to Europe in return for 3 billion euros (S$4.6 billion) of financial aid for the 2.2 million Syrian refugees it is hosting, as well as renewed talks on joining the 28-nation bloc.