LAGOS, Nigeria -- Militants in Nigeria's oil-rich southern delta ambushed a group of police officers as they sat stranded in a boat in the region's winding creeks, leaving at least 12 "missing" in the most serious attack in months in a region vital to the country's finances, a police spokesman said Saturday.

It remains unclear who carried out the attack Friday near the village of Azuzama in Bayelsa state, though a major militant group only days earlier threatened it would resume attacks following the arrest and imprisonment of its leader in South Africa. However, like most violence in the Niger Delta, the motives behind the assault remained as murky as the muddy creek waters flowing through a region vital to U.S. oil supplies.

The militants attacked the contingent of police officers, who were on an unspecified special assignment, after their boat broke down near the village, Bayelsa state police spokesman Alex Akhigbe said.

"Their boat developed some mechanical fault," Akhigbe told The Associated Press late Saturday night. "While they were trying to fix the boat, they were ambushed by some militants."

Akhigbe declined to say what the officers' special assignment was. He also declined to say whether authorities believed the officers had died in the attack, though he said a special rescue team had been assembled for the "recovery" of the officers.

A federal police spokesman in Nigeria's capital, Abuja, did not respond to request for comment.

Nigeria's Niger Delta, a region of palm groves and swamps about the size of Portugal, has been producing crude for foreign oil companies for more than 50 years. Despite the billions of dollars flowing into Nigeria's government, many in the delta remain desperately poor, living in polluted waters without access to proper medical care, education or work. The poor conditions sparked an uprising in 2006 by militants and opportunistic criminals who blew up oil pipelines and kidnapped foreign workers.

By 2009, most gave up their weapons as part of a government amnesty program that saw ex-fighters offered cash payments and the possibility of job training. However, while some militant leaders now live in the country's capital and enjoy the privilege of diplomatic passports, few in the delta have seen the promised benefits and sporadic kidnappings and attacks continue.

Even more worrying for foreign oil companies is the continued tapping of their pipelines and the theft of crude oil from them. Royal Dutch Shell PLC, the dominant company in the region, blames the oil theft for major environmental damage and says criminals steal as much as much as 150,000 barrels of crude a day. Shell produced about 800,000 barrels of oil a day in 2011. The International Energy Agency estimated that the theft of crude oil alone in Nigeria cost the country about $7 billion a year.

Nigeria, which produces about 2 million of barrels of oil a day, is a top crude oil supplier to the U.S.

Akhigbe declined to say whether police had any suspects in the attack. On Wednesday, the one-time main militant group in the region, the Movement for the Emancipation of the Niger Delta, threatened to launch a series of attacks over the arrest and sentencing of Henry Okah, a gunrunner long suspected by officials of being a leader of the group. However, analysts believe the militant group has lost much of its operational capability after Okah's arrest over the 2010 car bomb attack in Abuja that killed at least a dozen people.

• Read more articles by JON GAMBRELL

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