“The Global Cheyenne, a construction barge working in the Okan field, was attacked by unknown armed persons in speed boats,” US major Chevron said.
“Four expatriate American hostages were taken from the vessel and some government security forces suffered injuries during the attack.”
The barge, operated by US contractor Global Industries, was laying pipelines for Chevron.
Oil supplies were unaffected. Chevron ships about 160,000 barrels per day from the nearby Escravos terminal.
No group has claimed responsibility for the latest attack.
On Tuesday, rebels blew up three oil pipelines in the delta, forcing Italian oil major Eni to halt its output of 150,000bpd, according to various media reports.
There has been a surge in violence since April’s general elections, which were widely condemned as fraudulent by international observers.
The Movement for the Emancipation of the Niger Delta (MEND) claimed it bombed the pipelines to embarrass President Olusegun Obasanjo in his last days in office but has denied responsibility for kidnapping the Americans.
MEND’s 18-month campaign of attacks has forced the closure of almost a third of Nigeria’s oil capacity, and the group has threatened to launch more attacks before Obasanjo steps down on May 29 for president-elect Umaru Yar’Adua.
MEND has urged oil workers to leave the region and is aiming to completely halt Nigerian oil exports.
While MEND claims to be fighting to increase regional autonomy and control over the delta’s petroleum wealth, most kidnappings are probably apolitical criminal acts conducted by criminal gangs, according to analysts.
Most of the 100 or so foreigners abducted this year were freed after their employers illegally paid ransoms.
On Tuesday, three South Koreans and eight Filipinos employed by Daewoo Engineering and Construction were released after five days in captivity.
Their kidnappers had reportedly asked for a ransom of $US1 million ($A1.2 million), although it is not clear how much was actually paid.
The abduction of the Chevron workers takes the total number of foreigners now in captivity to 13.
