There have been five separate kidnappings of foreign oil workers in the country's Delta region in the past two weeks, and along with attacks on oil facilities by militants, oil output has fallen by 25 percent, the BBC reported Wednesday.
Two Norwegians and two Ukrainians were freed unharmed Tuesday after being seized on Aug. 9 from a moored supply vessel. Monday, four other workers were seized and five were released.
"Nigeria is a signatory to international conventions which categorizes hostage-takers and their accomplices ... as terrorists to be hunted down wherever they go," Obasanjo said.
The president said he had ordered 24-hour patrols in the Delta's creeks and swamps and threatened action against oil firms who paid ransoms for their workers.
Militants are fighting for more employment and local control of the region and its resources, the report said.