Militants holding nine foreign hostages in southern Nigeria destroyed an oil pipeline Monday and blew up a boat in violence that has cut about 20 percent of crude production in Africa's oil giant.

The Movement for the Emancipation of the Niger Delta said it attacked a Shell-operated oil-pipeline switching station known as a "manifold" and a military houseboat in the oil-rich southern region. "Both were destroyed with explosives," the group said in an e-mail.

Shell Spokeswoman Lisa Givert confirmed the oil-pipeline attack and said the houseboat was abandoned when the attackers blew it up. It was unclear who owned the boat.

The militants announced no casualties, and said the Nigerian sailors fled when the assailants attacked the boat aboard which sailors based in the region live. Military officials in the region could not be reached for comment.

The West African nation is reeling from weekend attacks in which militants blasted oil and gas pipelines and sabotaged a key oil loading terminal belonging to Royal Dutch Shell. That and an earlier attack has forced the company to halt the flow of about 455,000 barrels a day — about one-fifth of daily output in Africa's top crude producer.

The weekend violence sent crude prices higher in international markets.

April Brent crude futures rose $1.47 to $61.36 a barrel on London's ICE Futures exchange. Trading on the New York Mercantile Exchange was closed for the Presidents Day holiday. Oil prices had jumped more than $1 and settled near $60 a barrel Friday over supply concerns.

The militants claiming the attacks say they have kidnapped nine foreign oil workers and threatened to spread the violence further across the restive south, and said they would kill President Olusegun Obasanjo if he entered the region.

"We are going to continue with the destruction of oil facilities in Delta State while concluding arrangements for our wider attacks on the entire region," the group said. "We are declaring a war on Obasanjo. We will attack and kill him should he venture into the Niger Delta for any reason."

Violence and sabotage of oil operations have been common in the oil-rich Niger Delta for the past 15 years amid demands by the region's impoverished communities for a greater share of the oil revenue flowing from their land.

The militants, who say they are fighting for the same cause and the freedom of imprisoned ethnic Ijaw leaders, launched a series of pre-dawn attacks Saturday that shook the nation's volatile oil industry.

In one assault in the swampy delta's Forcados estuary, dozens of armed militants seized nine foreigners after storming a barge belonging to the Houston-based oil services company Willbros Group Inc., which was laying pipeline for Shell.

The hostages include three Americans, two Egyptians, two Thais, one Briton and one Filipino, militants and Willbros officials said.

Responding to local rumors they planned to execute the hostages, militants said in an e-mail to AP they had not decided what to do with them.

Nigeria is Africa's leading oil exporter and the United States' fifth-largest supplier, usually exporting 2.5 million barrels daily. Hostage takings are also a common occurrence in the volatile delta, but most are released unharmed. Last month, militants held four foreigners for 19 days before releasing them unscathed.