The National Maritime Authority (AMN) said in a statement that the incident occurred "during a search and rescue operation for migrants off the coast of Lesbos" and that there were six people on board, who were "safely rescued and are in good physical condition."

The Portuguese police force also emphasised that "an investigation has already been initiated to determine the causes of the incident."

Greek television station ERT, cited by Europa Press, reported that the Portuguese vessel sank around 6:00 PM (4:00 PM in Lisbon) during a search operation for four migrants in a pirogue (a boat made from a single tree trunk) crossing the Turkish coast.

A Greek coast guard source told Agence France-Presse (AFP) that the vessel, which is part of the European Border and Coast Guard Agency (Frontex), sank after hitting a reef near the island of Lesbos.

The boat's crew consisted of five Portuguese and one Greek, he added.

"It was participating in the rescue of migrants in the area, in almost gusty wind conditions," he explained.

The operation was launched after 36 migrants, spotted by police on a beach in Lesbos, reported to authorities that other members of the group were missing at sea.

The Greek Coast Guard also stated that searches for the migrants are ongoing.

Frontex's role is to assist European Union (EU) and Schengen Area countries in managing their external borders.