Militants who seized nine foreign oil workers in a string of attacks across Nigeria's troubled delta region threatened Sunday to step up assaults by firing rockets at international oil tankers.

The Movement for the Emancipation of the Niger Delta, which claims to be fighting for a greater local share of Nigeria's oil wealth, claimed responsibility Saturday for a series of raids, including one in which militants abducted three Americans, two Egyptians, two Thais, one Briton and one Filipino. The violence cut the West African nation's crude oil exports by 20 percent.

A man identifying himself as a commander of the movement told The Associated Press by telephone his group was poised to escalate the violence by firing rockets at crude oil tankers offshore.

"We'll use our rockets on the ships to stop them from taking our oil," said the man, who gave his name as Efie Alari. His identity could not be independently verified, but the call came from a number previously used by the group.

The movement has said the attacks were in retaliation for assaults this week by military helicopters.

The raids began before dawn, when more than 40 militants overpowered military guards and seized the foreigners from a barge belonging to Houston-based oil services company Willbros, which was laying pipeline for Shell, a Willbros official said on condition of anonymity because he was not authorized to speak to the media.

In Houston, Willbros spokesman Michael Collier confirmed that nine employees had been taken.

"We have not had any communication with those involved. Right now, we're in the process of contacting the families. The well-being of our people is foremost and we're trying to keep this situation under control as best we can," he said.

In Washington, State Department spokesman Noel Clay called for the hostages' unconditional release and said: "We're working with the Nigerian government and talking with them about this."

In other, apparently coordinated violence, militants blew up a major Shell crude oil pipeline near a facility by the western delta's Chanomi Creek, Shell official Donald Boham said.

Militants also claimed they destroyed a state-run pipeline that feeds gas from the Escravos gas plant in the delta to the country's commercial capital, Lagos. That attack could not be independently confirmed.

No casualties were reported.

President Olusegun Obasanjo held an emergency meeting late Saturday with security chiefs, governors from the oil region and the head of Shell's operations in Nigeria. Obasanjo, a government statement said, "wishes to assure all stakeholders in the region that everything possible is already being done to secure the speedy release of the hostages through dialogue."

The violence tooks its toll on oil exports in Nigeria, Africa's leading oil exporter and the United States' fifth-largest supplier, that normally producing 2.5 million barrels a day.

A fire was put out on a Royal Dutch Shell platform that loads the company's tankers in the western delta, but the Forcados terminal's normal operations could not continue, halting the flow of 400,000 barrels a day.

Shell said it had also evacuated an oil platform off its Atlantic coast as a precaution, shutting off an additional 115,000 barrels a day.

On Friday, Shell shut down a facility pumping 37,800 barrels daily after a fire at a nearby oil well. The firm has yet to restore 106,000 daily barrels lost when a major pipeline supplying the Forcados terminal was hit last month by a similar wave of attacks and hostage takings.

Oil prices jumped more than $1 and settled near $60 a barrel Friday on supply concerns sparked by a militant threat to wage war on foreign oil interests.

The militants have accused foreign oil companies of providing their helicopters and air strips for military operations in the oil region. They said they would now target all helicopters in the delta, including civilian aircraft.

On Saturday, the militants reiterated warnings that foreign oil workers must leave the Niger Delta, saying the expatriates were "caught up in a war, and the Nigerian government can do nothing to guarantee the security of anyone."

Militants identified each of the foreigners kidnapped Saturday by name. Thai Foreign Ministry spokesman Sihasak Phuangketkeow on Sunday confirmed two names on the list: Somsak Mhadmho, 43, and Arak Suwanna, 33, both of Bangkok. Sihasak said both men were married and their families have been informed.

Britain's Foreign Office said the British man kidnapped was John Hudspith of southern England. Clay confirmed three American oil workers were among those taken hostage.

Last month, militants held four men — from the United States, Britain, Bulgaria and Honduras — for 19 days before releasing them unharmed.

Over the past two decades, oil companies in the Niger Delta have faced frequent disruptions to their operations, including protests, pipeline sabotage and kidnappings.

Most hostages, however, have been freed within days after ransom payments. They are rarely harmed.