Nigerian militants say a Lebanese worker they had been holding was not released but escaped with outside help.
In a statement Thursday, the Movement for the Emancipation of the Niger Delta said the escape plan was hatched by the Italian oil firm Agip and the Bayelsa state government.
The militant group says Agip and the state government would "pay a hefty price" for their action.
Agip's parent company Eni announced the Lebanese worker's release Wednesday.
The man was kidnapped in December along with three Italian co-workers, one of whom has since been freed.
Over the past year, gunmen in the Niger Delta have seized more than 100 hostages. Most have been released unharmed.
The militants say they want impoverished villages to get a greater share of the region's oil wealth.
Nigeria is Africa's largest oil producer. The country pumped about 2.6 million barrels a day until last year. However, the militant activities in the Niger Delta have, at times, cut production by almost 25 percent.
Some information for this report was provided by AP and Reuters.