Gunmen have boarded a Greek-owned tanker anchored near a Nigerian port, killing one crew member and taking three others hostage, officials say.
The Kalamos was attacked while it was waiting to load at Qua Iboe, an oil terminal in south-eastern Nigeria.
The pirates killed a Greek deputy captain of the ship, and took two Greeks and a Pakistani citizen hostage, according to the Greek government.
The Gulf of Guinea in West Africa is regarded as a new centre of piracy.
The International Maritime Bureau recorded 33 incidents of piracy and armed robbery in the area between January and September last year, according to the AFP news agency.
Greece's deputy shipping minister, Thodoris Dritsas, said the "foreign and shipping ministries have taken the necessary actions for the rescue of the hostages", in a statement quoted by the Reuters news agency.
The remaining 19 crew members -which included seven Greek seamen, according to our information- are safe.
The Maltese-flagged Kalamos had travelled from China without any cargo.
The ship remains anchored off the coast of Nigeria.
The rest of the crew, which included seven Greek seamen, according to our information, is safe.
M / T "Kalamos" flag Malta, which was awaiting loading in the sea area of Qua Iboe Nigeria, with twenty-three (23) members of the crew, including ten (10) Greek.
This ship came from China bound for Nigeria.
During the attack, the Greek Chief Officer board was fatally injured and pirates kidnapped three (03) crew members, of which two (02) Greeks.
According to collected to date information the M / T "Kalamos" remains securely moored above sea area, while the incident has already informed the State Department for its own within reason competence, further actions. "
It is reported to have asked the Greek consular authorities in the country to speed up the body of transport processes in Greece and to investigate whether any of the crew members wishing to return to the necessary steps.