The problems in Nigeria's Niger Delta region recently prompted Shell Oil to declare it could not meet contracts because of forces outside its control. Chevron took the same action Tuesday, The Wall Street Journal reported.

The activities of the Movement for the Emancipation of the Niger Delta have contributed to the run-up in oil prices. The recent attacks have cut the country's oil production in half.

"More of such attacks will continue as long as our people continue to be

shortchanged in the affairs of the Nigerian state," Cythia Whyte, a spokeswoman for the Joint Revolutionary Council, said in a statement posted online. "Violence begets violence. If they attack us, then we too would follow that option. No one has the monopoly of violence. Rebellion against tyranny is righteousness before God. We have suffered enough."