Niger Delta - Shell's Nigerian subsidiary said on Friday it was investigating a claim by the Niger Delta Avengers that it had struck another pipeline in the restive oil-producing south.
"We are investigating reports of an attack on our pipeline in the western Niger delta," Shell Petroleum Development Company (SPDC) spokesman Precious Okolobo told AFP, without elaborating.
The Niger Delta Avengers (NDA), blamed for a wave of bombings on Nigeria's oil infrastructure since the beginning of the year, earlier claimed responsibility for the attack.
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"At 3:00 am today (0200 GMT Friday), @NDAvengers blow up the SPDC Forcados 48" export line," it said on its Twitter account.
The attack was carried out because Shell had refused to heed earlier warnings not to repair the pipeline, which was damaged in February, it added.
The NDA, which says it is seeking a fairer share of Nigeria's oil wealth for the Niger delta people and has asked oil majors to leave the region, also said it had struck Agip facilities.
"At about 3:30 am today (0230 GMT Friday), strike team blow up the Brass to Tebidaba crude oil line in Bayelsa (state)," it said on Twitter.
Desmond Agu, commandant of the Nigerian Security and Civil Defence Corps (NSCDC) in the state, confirmed the second attack and said the pipeline had been shut down.
"We have dispatched our men and we have intensified patrols in the area. We are working round the clock to protect critical infrastructure in Bayelsa," he added.
There was no immediate word from Agip's parent company, Eni of Italy, when contacted by email.
The NDA has been attacking facilities operated by subsidiaries of Shell, Eni, US firm Chevron, and the state-run Nigerian National Petroleum Corporation (NNPC).
The army said on Thursday that six people, including two soldiers, were killed when militants attacked a boat belonging to the NNPC in the region, but the NDA denied involvement.
The deadly attack happened at about 5:50 pm (1650 GMT) on Wednesday in the Warri area of Delta state, which has seen a spate of attacks recently.
That followed two claimed NDA attacks on two supply pipelines in Bayelsa state on Wednesday and a warning it would bring oil production to a standstill in Nigeria unless its demands were met.
The upsurge in violence has seen Nigeria's oil production fall to 1.4 million barrels per day, well below the budgeted for 2.2 million bpd.
The Avengers are believed to have sympathy for a former oil rebel leader who commanded militants in the region in the 2000s and who is now wanted on money laundering and corruption charges.
The Nigerian military, which has slammed the NDA as "economic terrorists", has deployed gunboats and fighter jets to the creeks and swamps of the delta in search of the militants.
Nigeria's President Muhammadu Buhari last week ordered enhanced security in the delta, adding: "We have to be very serious with the situation because if threatens the national economy."