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.

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.

It is unknown if the employees were locals or foreigners working with Afren, which explores eight oil fields in Nigeria with local partner companies. French Foreign Ministry spokesman Bernard Valero said Monday that his nation had "indications" about "the possible presence of two French citizens among the people captured" — though he offered no official confirmation.

In a statement, Afren said the attack occurred at a new rig doing survey work at the Okoro oil field, which sits about 7 miles (11 kilometers) off the coast of Nigeria's Akwa Ibom state. 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. An update will be provided in due course."

Nigerian navy vessels often offer security for such offshore rigs, however there were no vessels immediately in the area. Capt. Kabir Aliyu, a navy spokesman, said two navy boats rushed to the scene and sailors shot at the vessels carrying the gunmen and hostages as they sped away.

Security officials have no immediate suspects in mind, but they continue to investigate the incident, he said.

"The Nigerian Navy is doing its best," Aliyu said. "We are sending more ships to provide security for the oil companies operating in the area."

Afren operates in the Okoro field with the private Nigerian firm Amni International. In financial reports, Afren has said the field produces about 18,800 barrels of oil a day, with an estimated reserve of 24.8 million barrels.

Shares in Afren dropped 6.09 percent in trading Monday on the London Stock Exchange.

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.

Associated Press writers Bashir Adigun in Abuja, Nigeria and Jamey Keaten in Paris contributed to this report.

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Afren PLC: http://www.afren.com