Abbey said no group or individual had yet claimed responsibility for Thursday's abductions and no ransom has been demanded so far.

Hundreds of mostly foreign and local oil workers have been kidnapped in the volatile Niger delta since 2006. Most have been released unharmed, and some were freed only after ransom payments.

Fighters, who demand that the federal government send more oil-industry funds to Nigeria's southern region, have launched numerous attacks on oil-installations in the area and fought government troops since January 2006.

On March 31, a local employee of French oil group Total was abducted on his way to work in Port Harcourt.