LAGOS, Nigeria (CNN) -- A rebel group in Nigeria said it sabotaged two oil pipelines in southern Nigeria on Monday.
The Movement for the Emancipation of the Niger Delta said "detonation engineers backed by heavily armed fighters" sabotaged two pipelines about 1:15 a.m. Monday.
The rebels said they believe that both pipelines belong to the Shell Petroleum Development Company -- one in the the city of Kula, Nigeria and the other in Rumuekpe, Nigeria.
Shell said a helicopter flew over the area and confirmed that parts of its large Nembe Creek "trunk line" was damaged.
"We are working to ascertain the extent of the damage, and have shut in some production to limit the amount of crude that will spill into the environment," said Shell spokeswoman Caroline Wittgen.
The company was still checking on its Rumuekpe pipeline, she said.
Nigeria is the fourth-largest supplier of oil to the United States, and attacks by rebels have helped fuel the year-long spike in crude oil prices. It's one of many factors pushing up the price of gas in the United States, where one in every 10 barrels of oil comes from Nigeria.
The Movement for the Emancipation for the Niger Delta has targeted foreign oil companies since 2006. It has bombed pipelines and kidnapped hundreds of foreign oil workers, typically releasing them unharmed, sometimes after receiving a ransom payment.
MEND hopes to secure a greater share of oil wealth for people in the Niger Delta, where more than 70 percent of the population lives on less than a dollar a day.
Its attacks on oil facilities have taken a toll.
"Anytime a pipeline is affected, anytime any production gets shut down, you see oil prices jump up one or two dollars a barrel just because there is no slack in the system," said Jim LeCamp, a senior vice president with RBC Wealth Management, which manages assets for wealthy clients worldwide.
Exxon and Shell are two of several companies that had been extracting two million barrels of oil a day in Nigeria. Recent rebel attacks on oil pipelines in the Niger Delta have cut overall production by roughly 10 percent -- meaning 200,000 fewer barrels of oil on some days.
That decrease in production comes at a time of increased demand from oil-hungry regions such as China, Russia and Latin America.
"Anytime there's a disruption there, it really affects the system," LeCamp said in a recent interview with CNN.
CNN's Christian Purefoy contributed to this report.