Gunmen in Nigeria have attacked an oil rig and seized five workers after a recent lull in such raids. Afren, the company which operates the offshore rig, said two workers had been wounded. The nationality of those taken hostage is not clear. The raid came as an e-mail was sent to journalists warning of new attacks on oil installations in the Niger Delta. Violence in the oil-producing region had subsided after the main militant group accepted an amnesty last year. London-based Afren PLC said the gunmen had also attacked a support ship but that both the boat and the rig in the Okoro field off the coast of Nigeria's Akwa Ibom state were now under its control. The wounded pair have been flown by helicopter for medical treatment, it said. The company did not provide any further details of the attack but said drilling operations had been suspended. Violence in the Delta region had caused a sharp fall in Nigeria's oil output until the amnesty offer saw thousands of gunmen lay down their arms. But a faction of the main militant group is accused of carrying out twin car-bombings in the capital, Abuja as Nigeria was celebrating 50 years of independence on 1 October. Source: BBC