The barge, operated by U.S. contractor Global Industries, was laying pipelines for the U.S. oil company at its Okan oilfield, said the sources, declining to be identified.

"Armed men on two speed boats with rifles and RPGs (rocket-propelled grenade launchers) boarded (the barge). Four American personnel were abducted," said one source.

A Chevron spokesman declined to comment. It was not immediately clear if oil exports were affected.

On Tuesday, Nigerian rebels blew up three oil pipelines in the Niger Delta, forcing Italian oil company Eni <ENI.NI> to halt production of 150,000 barrels per day (bpd) feeding its Brass export terminal, a source at Eni said.

The Movement for the Emancipation of the Niger Delta (MEND), which has forced the shutdown of almost a third of Nigeria"s oil capacity, said the attack was designed to embarrass President Olusegun Obasanjo in his last days in office.

MEND, which has waged attacks for 18 months, threatened to launch more in the world"s eighth-largest crude exporter before Obasanjo steps down for President-elect Umaru Yar"Adua on May 29.

Raids on oil installations and abductions of foreign workers have become frequent in the delta, a maze of mangrove-lined creeks that is home to Africa"s largest oil reserves but where the majority of people live in poverty.

MEND says it is fighting for regional control over the delta"s oil wealth but the majority of kidnappings are motivated by criminal gangs seeking ransoms.

Three South Koreans and eight Filipinos were freed on Tuesday after five days.

The abduction of the U.S. oil workers takes the total number of foreigners still in captivity to 13.

Thousands of foreign oil workers have fled Nigeria because of militant attacks since Feb. 2006.

PIN/ REUTERS