A Turkish maritime company says six crew members of a tanker ship who were captured by pirates off the coast of Nigeria have been released.

The pirates boarded the Malta-flagged M/T Puli on April 11 and captured six of the ship's 14 crew members, including the captain. Fehmi Ulgener, a lawyer for the firm Kaptanoglu Denizcilik, said Tuesday the freed crew members are in good health.

He did not elaborate on the circumstances of their release or say whether a ransom was paid.

The United States warned Monday during a U.N. Security Council meeting that piracy and armed robbery are increasing at an alarming rate in the Gulf of Guinea, saying industry experts have reported at least 32 attacks off the coast of Nigeria alone so far this year.