Since 2005, attacks have cut 20% to 30% from the nation's oil output. But recently, the oil conflict has combined with labor strikes and other problems to reduce output by 1 million barrels a day, down to an average 1.8 million barrels a day.
The decline in West African production represents a fraction of U.S. consumption, about 21 million barrels a day. But it has a powerful effect on oil markets at a time when output from Mexico and Venezuela is falling, and Iraq's production languishes below prewar levels.
"The result of this is a much tighter market, in terms of balance between supply and demand, than you have had for decades," Yergin, the oil expert, testified before Congress last week.
The effects of the conflict in Nigeria were clear last weekend, as the Saudis announced at an international oil conference in Jidda that they would increase output.
Ordinarily, that might have set off a slide in prices; instead, they rose in part because of concern over the rebel raid on Shell's Bonga offshore platform.
The disruptions in Nigeria have led to calls for the United States, Britain and other world powers to take concrete steps toward a peace deal between the Nigerian government and the rebels.
Some senior U.S. military and North Atlantic Treaty Organization officials have urged the Bush administration to build up Nigeria's weak coastal navy, or even use U.S. naval vessels along with other regional forces to patrol Nigerian waters, said officials who spoke about the internal discussions on condition of anonymity.
U.S. officials say they are taking limited steps to strengthen the Nigerian military. But, despite intermittent reports in the Nigerian news media, they said the United States has no intention of intervening militarily.
The Bush administration has tried to spur a political settlement with the rebels. But U.S. officials have complained that the efforts have met with resistance, in part because civilian and military officials are content with the situation, which provides revenue and helps maintain their influence.
"All the actors in the delta are benefiting from the status quo," said one U.S. official, who declined to be identified because of the diplomatic sensitivity of the issue. "They'll do their best to stop any peace process; it threatens their business."
