Reports from Nigeria's southern delta region say militants kidnapped 25 workers during Monday's attack on an oil-industry boat convoy.
About 70 gunmen traveling in speedboats attacked the military-escorted convoy, killing at least five soldiers.
A group of local separatists later released a statement claiming responsibility, and said the abductions were aimed at obtaining the release of a separatist leader. The group said it killed the soldiers in revenge for attacks by troops on local communities.
Oil-industry giant Royal Dutch Shell, which employs the kidnapped contractors, has not commented on the abductions.
Militant attacks on Nigeria's oil industry, Africa's largest petroleum producer, have cut production by nearly a third this year.
Despite the region's vast oil wealth, local people remain impoverished. Militants have demanded a greater share of oil revenue, however many recent kidnappings are believed to have been carried out strictly for ransom money.
Some information for this report was provided by AFP, AP and Reuters.
