Abuja, Nigeria ? Security officials said militants kidnapped several foreign oil workers Tuesday in Nigeria’s oil-rich southern delta region, a day after 25 Nigerians were taken hostage in an attack on a military convoy escorting oil workers.

Police officers in the town of Ekit, outside the main city of Port Harcourt, said four to seven foreigners were seized in the raid there, but they gave no details.

The workers’ nationalities were not immediately known.

Attacks by militant groups over the past year have cut nearly a quarter of Nigeria’s usual output. Nigeria is Africa’s largest petroleum producer and the fifth-largest supplier of crude oil to the United States, but most people in the southern oil region remain deeply impoverished, fueling the armed dissident groups.