Our people went into negotiations with the youths and the hostage-takers agreed to vacate the rig last night

Shell spokesman

The group had been held captive by a group of militant youths on a rig almost 100km off Nigeria's coast.

News of the incident was only released after the incident was resolved.

The BBC correspondent in Nigeria, Dan Isaacs, says incidents such as this are not uncommon in the country's troubled oil-producing region, but oil companies rarely provide details of kidnap situations such as this until they have been resolved, for fear of jeopardising negotiations.

He says it is now clear that the gang required motor boats to travel the substantial distance out to sea.

Once there, they took control for three days while oil industry officials negotiated for the release of those on board.

The offshore rig, Trident, is owned by Houston-based Transocean Sedco Forex. It was drilling on behalf of oil giant Shell.

According to the Reuters news agency, the situation was resolved when community elders were called in by Shell to ask the youths to leave.

Shell officials said nobody was injured.

"Our people went into negotiations with the youths and the hostage-takers agreed to vacate the rig last night [Sunday], but it was late so they couldn't leave," said a spokesman.

Militant campaign

In June last year, oil workers working for the US oil company Chevron were seized by youths reportedly supporting the struggle in the Niger Delta area for a greater share of Nigeria's oil wealth.

The 24 workers, mostly Nigerian workers, were later released unharmed.

Militants in the Niger Delta have frequently carried out acts of sabotage and taken foreign workers hostage in order to draw attention to their cause.

Shell has been at loggerheads with the militants, who accuse the company of not doing enough to protect the environment and of not investing in the development of the area, where many residents live in abject poverty without paved roads, electricity or running water.

Shell and Chevron are two of the major oil companies operating in the Delta region, where most of the country's oil is drilled.

Nigeria is the world's sixth-largest oil producer.