Gunmen attacked an oilinstallation in Nigeria's restive southern oil heartland and seizedfive expatriate workers, private security officials said. Police has been investigating a reported incident, but had no details. Two private security officials involved in protecting oil company staff in the Niger Delta said five expatriate workers had been reported seized. Both spoke on condition of anonymity, citing company policy prohibiting speaking to journalists.The spokesmen for a top militant gang, whose one-month cease-fire ended Tuesday, said their group was not responsible.Criminal kidnappings have become common in the region where the crude in Africa's biggest producer is pumped.More than a dozen foreigners are currently in captivity.