Armed men have attacked a pipeline operated by the US giant Chevron in southern Nigeria, but it was not immediately clear if production had been affected, industry and police sources said.

"The pipeline supplies crude to the Escravos terminal. It was attacked last night," an oil industry source said on Saturday.

No group has claimed responsibility for the attack, according to a senior police officer. Security agents are investigating the incident.

Oil pipelines, facilities and personnel of oil majors operating in Nigeria's restive oil-rich Niger Delta have been targets of violent attacks in the past three years. 

The oil giant Shell said on Friday that it had contained a spill caused by sabotage on its Adibawa delivery line in the southern Bayelsa state. Last week, armed men attacked Chevron's Robert-Kiri flowstation, killing a Nigerian navyman.

Since January 2006, attacks have cut Nigeria's oil output by more than one quarter. Nigeria's total production currently stands at around two million barrels a day against 2.6 million barrels in 2006.