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 between four and seven foreigners were seized in the raid there, but they gave no details.

A top Nigerian security official confirmed the kidnapping, but also had no details. All the security officials spoke on condition of anonymity, citing prohibitions on speaking with the media.

The workers' nationalities were not immediately known.

CountryWatch: Nigeria

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.

Abductions are common in the volatile delta, with most captives released unharmed.

Nine of the 25 workers abducted Monday in the attack on the convoy were released Tuesday, said Eurwen Thomas, a Royal Dutch Shell PLC spokeswoman. She had no details on the remaining 16 hostages — all oil workers — taken Monday.

During the attack, carried out by about 70 militants, four soldiers and a civilian working for the military were killed as the fighters sank two military patrol boats, army spokesman Maj. Sagir Musa said.

Bisi Ojediran, a spokesman for a Shell-led partnership that pumps most of Nigeria's crude, said no oil workers had been killed or injured in the attack.

In a separate incident Tuesday, militants attacked a boat carrying oil-service workers in the same area, wounding several soldiers, a private security official said. No hostages were taken.

The official spoke on condition of anonymity due to company rules against speaking to the media. A military official, also prohibited from speaking to reporters, confirmed an attack.

A group calling itself a coalition of militant groups in the Niger Delta region claimed responsibility for Monday's attack. The coalition demanded the release of imprisoned militant leader Mujahid Dokubo-Asari and said the action was taken in revenge for attacks by soldiers on local communities.

However, an e-mail from the Movement for the Emancipation of the Niger Delta, one of the groups the coalition claims to represent, denied responsibility for the attack.