LAGOS, Nigeria — Gunmen in speedboats kidnapped five workers and wounded two others in an attack Monday at an oil rig operating off the coast of Nigeria's troubled southern delta, officials said.

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A security source in Nigeria said those kidnapped were believed to be two U.S. citizens, two French nationals and one Canadian national.

The French Foreign Ministry said it had received information suggesting two French citizens were among those taken and was investigating.

The attack on the newly installed rig operated by London-based Afren PLC and a nearby support ship comes as sporadic attacks continue in a region supposedly calmed by a government-sponsored amnesty program.

Afren said the attack occurred at a new rig doing survey work at the Okoro oil field, which sits about eight miles off the coast of Nigeria's Akwa Ibom state. Afren explores eight oil fields in Nigeria with local partner companies.

The company said the two workers suffered "wounds to the leg" in the attack and have been flown out by helicopter to receive medical treatment.

"We are working with the relevant authorities and the vessel and rig are both under the control of the company," Afren said. "Drilling operations on the rig are temporarily suspended."

Nigerian navy vessels often offer security for such offshore rigs, though it was unclear whether they had vessels in the area. A naval spokesman said Monday that he had no information about the attack.

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Militants in Nigeria's oil-rich Niger Delta began a campaign of kidnapping and pipeline bombings in 2006, upset over pollution and the region's endemic poverty despite 50 years of oil production.

Violence in the region dropped after a government-sponsored amnesty program last year offered militants the promise of job training and cash payouts. However, some have become disillusioned and others remain armed in the delta's winding creeks.

No group immediately claimed responsibility for the attack. The region's main militant group, the Movement for the Emancipation of the Niger Delta, has said it would carry out new attacks in the region after claiming responsibility for an Oct. 1 car bombing in the nation's capital Abuja that killed at least 12 and injured dozens more.

An e-mail message purportedly from the group on Monday warned militants would "launch a series of attacks on oil installations across the Niger Delta." However, the message came from an account previously not associated with the group.

On Sept. 22, gunmen attacked an offshore rig operated by Addax Petroleum, a subsidiary of Chinese state-owned oil producer Sinopec Group. The assailants kidnapped three French contractors from an oil-rig support vessel that appeared on the scene, as well as a Thai expatriate during the attack. The four remain missing.