At least twelve migrants including four children died early Friday when their boat capsized near a small Greek town on the Ionian Sea, the coastguard said.

"We believe the migrants are Syrian," a coastguard spokeswoman said.

A statement said the migrants were found on land, near an overturned eight-metre (26-foot) fibreglass boat that was still tied to the shore.

The state-run Athens News Agency said the children were three boys and a girl aged between three and six, adding police speculation that the boat sank because it was overloaded.

A diver found four of the victims still trapped under the boat, the coastguard said.

The migrants were found at Paleros, a small western mainland Greek coastal town near the island of Lefkada.

Officials corrected earlier information that had described the craft as an inflatable dinghy, and that had said the incident occurred just off Lefkada.

Fifteen survivors were taken to hospitals in the neighbouring city of Preveza and to Lefkada.

"We find it puzzling how this accident occurred as weather conditions in the area were excellent this morning, and it also happened near the coast," Lefkada Mayor Costas Aravanis told Skai TV.

Greece is one of the main ports of entry into the European Union for migrants and refugees fleeing war-torn and impoverished countries in Africa, the Middle East and the Indian subcontinent.

Refugee traffic has soared over the past year because of the ongoing war in Syria. Arrivals by sea have increased owing to stricter controls on the Greek-Turkish northern land border.

Europe s immigration policies have come under the spotlight after more than 400 asylum seekers drowned in October in two tragedies near another popular migrant port of entry, the Italian island of Lampedusa.