The Movement for the Emancipation of the Niger Delta, or MEND, said in an e-mailed statement that the targeted pipelines are operated by Royal Dutch Shell.
"In keeping with our pledge to resume pipeline attacks within the next thirty days, detonation engineers backed by heavily armed fighters ... sabotaged two major pipelines in Rivers state of Nigeria," Jomo Gbomo, MEND's press secretary, said in the statement, reported the Agence France-Presse.
The attacks ended the weeks-old cease-fire intended to give regional officials time to negotiate an agreement with the government. The cease-fire was called off in response to statements by Britain pledging to support the Nigerian government in the conflict with militants.
MEND has used pipeline sabotage, as well as kidnapping, for the last two years to try to force the Nigerian federal government to send more oil profits to the impoverished southern region where the crude is produced.
On Saturday, eight foreign oil workers were kidnapped at gunpoint by six unidentified men, but released unharmed hours later.
Shell Petroleum Development Co., the company's local unit, confirmed Monday an incident occurred around the Kula area of Rivers state.
"A helicopter overfly today confirmed that parts of SPDC's Nembe Creek trunk line were damaged in attacks," Shell spokesman Rainer Winzenried said in an e-mailed statement, reported Bloomberg News.
"We are working to ascertain the extent of damage, and have shut in some production to limit the amount of crude that will spill into the environment."
The company was not able to provide specifics on how disruptive the attacks will be to production.
Militant attacks have halted about 20 percent of Nigeria's oil production since 2006, contributing to crude prices hitting new records in international markets. Nigeria has the capacity to produce at least 2.5 million barrels per day, according to the Associated Press.
"We've known for some time that Nigerian production has not been reliable," Phil Flynn, vice president at Alaron Trading Corp told the Wall Street Journal.
"We're going to wait to see how much supply is affected, how bad it is and how long it's going to be down."
Oil prices rose slightly Monday in response to concerns about the Niger Delta, but investors are watching the situation closely to see if it worsens.
