Three hostages, two of them Americans, kidnapped off Nigeria when gunmen attacked a ship working with oil giant Chevron, have been freed, officials said Friday, but it was unclear if ransoms were paid.

"I can confirm two Americans were released last night," US embassy spokeswoman Deborah MacLean said, adding she had no further details on their release.

Chevron later confirmed that all three of the hostages taken in the November 17 incident had been freed, but also provided no further details.

A private security source had previously said the third hostage was a Mexican citizen, but Chevron declined to comment on their nationalities.

Eight armed pirates boarded the ship and attacked the crew, taking the three victims hostage, Chevron had said at the time. The ship, the MV C-Endeavour, belonged to US firm Chouest.

A 2009 amnesty deal led to a sharp decline in attacks in the country's oil-producing Niger Delta region, but sporadic incidents continue to occur.

Nigeria is Africa's largest oil producer.