They were taken from a pipelaying vessel owned by Texas-based Transcoastal, contracted to Conoil, a Nigerian oil company, the sources said.
Nigerian authorities and Transcoastal officials were not available for comment.
The raid came a day after armed men kidnapped a Polish engineer near the oil city of Warri, also in the Niger Delta, a vast wetlands region which is home to Africa's largest oil industry.
Abductions for ransom or to press political demands are frequent in the impoverished delta, where there is widespread resentment against an industry that has extracted billions of dollars in oil wealth but left most people living in poverty.
Oil production from Nigeria, the world's eighth biggest exporter, is down by about 700,000 barrels per day or almost a quarter because of an 18-month surge in violence.
Change in government
Unrest has peaked in the weeks running up to a change of government due to take place on Tuesday, when Olusegun Obasanjo, the president, hands over to Umaru Yar'Adua.
Armed groups demanding regional control over oil revenues, compensation for oil spills and freedom for two jailed leaders from the region, see the handover as an opportunity to extract concessions from the central government.
But crime flourishes in the lawless delta and many abductions are motivated by money.
About 100 foreigners have been kidnapped this year and released unharmed after their employers paid ransoms.