The five including foreigners were attacked on Sunday in the shallow-water Okoro oilfield off the Obolo local government area of Akwa Ibom.

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.

"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 the West African nation Nigeria said those kidnapped were believed to be two French nationals, two U.S. citizens and one Canadian national, though there has not been confirmation of this from the security services or Afren.

According to Afren, 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 were down more than 8 percent in London.

Adding, the company had been a second security breach at a support vessel but 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 until an amnesty was agreed last year.

MEND said in a statement emailed to media on Monday that it would launch a series of attacks on oil infrastructure in the coming days. 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. Reuters gathered.

Thousands of gunmen laid down their weapons under the amnesty deal brokered by President Goodluck Jonathan last year, including the main known field commanders of MEND.

But the militants were always highly factionalised and some armed gangs with grievances against foreign oil firms remain active, security sources say.

A resurgence of violence in the Niger Delta would be an embarrassment for Jonathan, who is the first Nigerian head of state from the Niger Delta and who faces a tough battle in presidential elections expected next April.