Pirates hijacked a Greek-owned oil tanker off Nigeria and kidnapped 19 Indian and Turkish crew members, highlighting a worrying trend that has seen many ships attacked off the West African coast.

The ship's Hong Kong-based managers said in a statement that the Nave Constellation was boarded late Tuesday by armed men after it departed the Bonny Offshore Terminal in "fully laden condition".

The management said all but one of the crew members taken hostage are Indian nationals.

Seven crew members remained on board and were told to take the ship to a safer position, the statement said. "All appropriate authorities" have been alerted, and "all the necessary action is being taken to secure their well-being and early release", said the statement. The families have been informed, it said.

The ship was not damaged, the statement said.

Many kidnapping cases involving foreign nationals have been reported in Nigeria before, with the abductors mostly demanding for huge ransoms from their countries.

In July this year, four Turkish nationals that had been abducted in Nigeria were rescued, with the country's police saying no ransom had been paid.

To tackle the ship attacks, the U.S. and 33 other countries earlier this year carried out weeks of maritime training in the Gulf of Guinea to improve safety against pirates and better monitor the coastline.

Attendees noted the need for better infrastructure, funding and coordination in a region where multiple languages including English, French, Spanish and Portuguese are spoken.