Gunmen seized four American workers as violence escalated in Nigeria's southern petroleum-producing region, a Chevron spokesman said Wednesday.

The attackers, carrying assault rifles and rocket-propelled grenades, stormed a vessel carrying the workers in the southern Niger Delta minutes before midnight Tuesday, two industry officials told The Associated Press, speaking on condition of anonymity because their company prohibits them from talking to the media.

Chevron Corp. spokesman Femi Odumabo said the four kidnapped workers were subcontractors with U.S. citizenship.

"Four American employees of Global Industry Ltd. were taken hostage," he said. "There is no current threat to production."

Another vessel in the same area, the Walvis 6, was also attacked and robbed. A crew member reported missing was later found hiding on the boat.

"This is just a piracy, robbery case," Odumabo said. Despite last week's attack, in which six Chevron employees were taken hostage, he said the company did not feel singled out as a target.

"It's not directed at any one company," he said.

Earlier Tuesday, militants staged coordinated attacks on three pipelines in the wetlands region, the most damaging assault on the country's vital oil infrastructure in over a year, marking a heightening of hostilities.

Nigeria is Africa's largest producer of crude, one of the top 10 exporters in the world and a leading supplier of oil for the United States.

The near-simultaneous blasts Tuesday followed the kidnappings of dozens of foreign oil workers last week, a sequence of events militants say is intended to shut down the continent's largest crude exporter.

Analysts believe armed groups are heightening the tempo of attacks in a bid to demonstrate their relevance ahead of this month's handover of power to a newly elected government. It was unclear if the Tuesday attacks were linked in the massive Niger Delta, roamed by various militant and criminal outfits.

The Movement for the Emancipation of the Niger Delta, the largest militant group in Nigeria's oil-rich region, claimed responsibility for the bombings and warned of more attacks ahead of a May 29 presidential inauguration meant to set up the country's first-ever handover of civilian power. In an e-mail message on Wednesday, MEND said it did not kidnap the four Chevron workers on Tuesday but was encouraging all groups to launch attacks.

"We have asked all groups to attack all facilities and oil workers," it said.

The militants claimed to have taken out the entire network of pipes leading to an Agip-operated terminal, which can export 200,000 barrels a day. Nigeria has a total production capacity of 3 million barrels per day, but protests and militant attacks had reduced oil production by around 680,000 barrels before Tuesday's bombings.

It was not immediately clear how much production had been cut by the attacks, since the Nigerian staff of Agip, a subsidiary of Italy's oil giant Eni SpA, were on strike for a second day and company representatives in Italy said they were unable to comment.

Nearly 100 foreign oil workers have been kidnapped since the beginning of the year. The kidnapping of the Chevron workers means that 32 foreigners have been abducted in the last 10 days.

Attacks in Nigeria often send ripples through markets already jittery over instability in the Middle East. The light crude that the west African country produces is much cheaper to refine than heavier oils and its proximity to North America cements its importance as the fifth-largest supplier of crude to the United States.

The attacks also come on the heels of Nigeria's presidential election in April, which put the country under an international spotlight following widespread evidence of vote-rigging.

The April polls, which President Olusegun Obasanjo's ruling party won by a landslide, were characterized by violence and did not take place at all in many areas of the impoverished Delta region. Many militant groups have issued statements saying they will not recognize the incoming government of the president-elect, Umaru Yar'Adua.