Mend said it attacked using machine guns and assault rifles
Armed men in the Niger delta of Nigeria say they have attacked an oil pipeline overnight, putting a two-month truce with the government in doubt.
A faction of the Movement for the Emancipation of the Niger Delta said it attacked the pipeline.
A spokesman said it was because the government was delaying peace talks due to the absence of ill President Umaru Yar'Adua, who is in Saudi Arabia.
Attacks have cost Nigeria millions in lost revenue over the years.
The faction said, in an e-mailed statement, that the "warning strike" was carried out by 35 men on five boats with assault rifles, rocket launchers and heavy-calibre machine guns.
It said the pipeline was in Abonemma, about 50km (30 miles) west of Port Harcourt.
Nigeria's military has not commented on the attack.
Peace talks were suspended when President Yar'Adua was hospitalised in late November in Saudi Arabia.
Mend said it would review the ceasefire within 30 days.
"While the Nigerian government has conveniently tied the advancement of talks on the demands of this group to a sick president, it has not tied the repair of pipelines, exploitation of oil and gas as well as the deployment and re-tooling of troops in the region to the president's ill health," it said.