A Nigerian court has charged 26 men with conspiracy to commit a maritime offence and attempting to illegally deal in crude oil after authorities accused their supertanker of sailing in Nigerian waters without authority.

The captain is an Indian national while the crew members are from Poland, India, Sri-Lanka and Pakistan, court papers showed.

At the request of Nigerian authorities, Equatorial Guinea detained the Heroic Idun, a vessel capable of carrying 2 million barrels of oil, on Aug. 17 for sailing without an identifying flag, fleeing from the Nigerian navy and sailing in Equatorial Guinean waters without prior authorization.

The 26 men, including the captain, appeared in a High Court in the Nigerian Rivers state capital Port Harcourt on Monday and Tuesday and the judge ordered them to be detained on their ship under the guard of the Nigerian navy.

The men, who all denied the charges, were accused of having "attempted to deal with crude oil within the Nigeria Exclusive Economic zone without lawful authority".

Nigeria said the vessel had not loaded any oil before the navy approached it, but said the ship made a false claim of a piracy attack, entered a restricted area without authorization and attempted to load crude oil illegally.

Oil theft has taken more than 400,000 barrels per day from Nigeria's oil output, hit state finances and knocked it from Africa's top exporter to number two, the state oil firm says.