Nigeria ? Gunmen attacked an oil rig Wednesday in Nigeria’s restive southern oil heartland and seized five expatriate workers, officials said.

Rivers state Police Commissioner Felix Ogbaudu said the attack came after midnight at a drilling rig on one of the countless islands in the vast riverine delta region where Africa’s biggest oil producer pumps its crude.

He said an Australian, two New Zealanders, one Lebanese and one Venezuelan were taken. Royal Dutch Shell said it owned the rig, but that there were no production cuts reported.

A top militant gang, whose one-month cease-fire ended Tuesday, said in an e-mailed statement that it was not responsible.