A baby, two children and a woman drowned on Thursday after their wooden boat and a coast guard vessel crashed during a migrant rescue operation off the island of Lesbos, the Greek coast guard said.

The boat, which the coast guard said was old, sank after the crash and 31 persons were rescued. The coast guard did not provide further details on the incident or the nationality of the victims.

Thousands of refugees - mostly from Syria, Afghanistan and Iraq - attempt on a daily basis to cross the AegeanSea from nearby Turkey, a short but perilous trip by boat, often in rough seas due to poor weather.

Almost 400,000 people have arrived in Greece this year, according to the U.N. refugee agency UNHCR, overwhelming the crisis-stricken government's ability to cope.

On Wednesday, a migrant woman and two children drowned when a boat carrying about two dozen people capsized off Lesbos, the coastguard said.

Frontex said on Tuesday that 710,000 migrants have entered the European Union so far this year without the right documents or via illegal entry points. Most have rapidly headed north towards Germany.

European leaders are holding talks in Brussels on Thursday over how to cope with the continent's worst migration crisis since World War Two.