The people on board the rig went inside and locked their doors

Dick Filgate, ChevronTexaco

Company official Dick Filgate said that about 40 youths had stormed the rig, originally holding 88 workers captive, including 22 foreigners.

However, 45 were released on Tuesday and negotiations were taking place to free the others. It is not known how many foreign workers are still on the rig.

The hostage takers are demanding contract jobs.

The incident happened on Sunday, although details have only just been released.

The rig is about 8km (5 miles) off the town of Escravos.

"They mobilised a bunch of boats and people, and occupied one of the drilling rigs," Mr Filgate said.

Workers not threatened

"The people on board the rig went inside and locked their doors."

He said the youths did not appear to be armed and the workers did not feel threatened.

ChevronTexaco spokesman Fred Gorell, speaking from San Francisco, said: "We are encouraged by an agreement that was reached today for 45 workers on the rig to leave.

"We hope negotiations will bring swift and peaceful resolution to the occupation."

Nigeria is one of the world's largest producer of oil.

Oil installations are often subject to hostage taking and acts of sabotage in Nigeria, where local communities feel they have been exploited by oil companies and successive Nigerian governments

Workers are rarely harmed by kidnappers, who usually demand jobs or other compensation.

In March 2000, 32 Shell workers at a natural gas plant in the Niger Delta were held hostage for five days by youths demanding the company improve a local road.