Pirates attacked a vessel operated by oil major ExxonMobil in the Niger Delta in southern Nigeria killing a crew member and injuring another, private security contractors working in the oil industry said.

Attacks by politically motivated rebels in the delta, who last year shut down a fifth of Nigerian oil production, have subsided since a new president took office in May but armed robbers and pirates still stage sporadic raids in the creeks.

Details of today's incident were sketchy, as is often the case with Niger Delta attacks, but three security contractors said about seven or eight gunmen in a speedboat boarded the Seamark vessel on the Bonny river, in Rivers state.

"One crew member was shot dead. They also shot the captain's cabin door and demanded money and in that process they shot and wounded another crew member," said one of the contractors. "They ransacked the vessel taking away a laptop and some radios. The vessel then continued on its way and is now secure at Onne," he said.

The victims were Nigerians, the sources said.

President Umaru Yar'Adua's envoys have tried to negotiate a ceasefire with rebel groups demanding local control over oil revenues for the impoverished communities of the Niger Delta. While talks have been going on, their attacks on oil facilities and kidnappings of oil workers have subsided.

But the line between militancy and crime is blurred in the delta, which remains prone to violent attacks such as armed robberies or kidnappings for ransom.

Nigeria, an OPEC member, is the world's eighth-biggest exporter of crude oil and disruptions to supply due to violence in the Niger Delta are one of the causes of oil's climb towards $100 a barrel.