LAGOS (Reuters) - Gunmen in Nigeria have attacked an offshore oil rig operated by exploration firm Afren, kidnapping five crew members including foreigners and injuring two others, the company said on Monday.
Nigeria's main militant group, the Movement for the Emancipation of the Niger Delta (MEND), made no immediate claim of responsibility but threatened to carry out new attacks on oil infrastructure in Africa's biggest oil and gas industry.
A resurgence of violence in the Niger Delta would be an embarrassment for President Goodluck Jonathan, who is the first Nigerian head of state from the region and who faces a tough battle in presidential elections expected next April.
The attack in the shallow-water Okoro field off Akwa Ibom state, Afren's biggest producing oilfield with an average output of 17,300 barrels per day, was one of the biggest of its kind since an amnesty brokered by Jonathan last year.
"Two crew members are stable after receiving wounds to the leg, and have been evacuated by helicopter to a shore-based clinic. It is believed that five crew members have been taken hostage," Afren said in a statement.
A security source in Nigeria said those kidnapped were believed to be two French nationals, two U.S. citizens and one Canadian national, though there was no confirmation of this from the Nigerian security services or Afren.
The French Foreign Ministry said it had received information suggesting two French citizens were among those taken and was investigating.
Afren said the High Island VII jackup rig had recently arrived in the Okoro field and was preparing to begin drilling. It said drilling activities were temporarily suspended. Afren shares fell as much as 9 percent in London.
The company said there had been a second security breach at a support vessel but that both the rig and the vessel were now under its control.
Akwa Ibom lies on the edge of the Niger Delta, a vast wetlands region home to foreign oil firms whose infrastructure had been repeatedly attacked for years, costing the country an estimated $1 billion (619 million pounds) a month in lost oil revenues at its peak.
The amnesty agreement brought more than a year of relative peace but the militants were always highly factionalised and new commanders for some of the region's armed gangs are emerging, security sources say.
MORE ATTACKS THREATENED
In an email to media, MEND warned of more attacks.
"In the coming days, our fighters will launch a series of attacks on oil installations across the Niger Delta," it said.
It also said it was holding one Thai and three French nationals who were kidnapped several weeks ago and had since been transferred to its custody.
The three Frenchmen were kidnapped from a vessel owned by French marine services company Bourbon on September 22 by gunmen in speedboats, an attack which was also off Akwa Ibom and similar in style to the latest strike on Afren.
Thousands of gunmen laid down weapons under last year's amnesty, including the main known field commanders of MEND.
The government says more than 7,000 ex-militants and their leaders have received retraining as part of the amnesty and has denied that the programme is breaking down.
MEND claimed responsibility for twin car bomb attacks near an independence day parade in the capital Abuja on October 1, a strike which killed at least 10 people. Jonathan said the group's name had been used as a cover.
Nigeria's secret service has said it has caught five suspects believed to have coordinated and carried out the attacks. Henry Okah, a suspected MEND leader, has been detained in South Africa in connection with the bombings.
An explosion rocked a government guest house in Delta state, one of the main oil-producing states in the region, last week, while two weeks ago saboteurs attacked an Agip oil pipeline, shutting 4,000 barrels per day of production.
Timi Alaibe, the president's adviser on the Niger Delta, said the Agip attack was carried out by criminal saboteurs rather than disgruntled former militants. The cause of the Delta state blast is still being investigated.
(Additional reporting by Austin Ekeinde in Port Harcourt, Julie Crust in London, Clement Guillou in Paris, editing by Tim Pearce)