Nigerian security forces said on Saturday they rescued 13 hostages kidnapped this week, including nine foreigners, and destroyed a key militant camp in the heart of Africa's biggest oil industry region.

Nigeria's main militant group has declared an "all-out war" and warned oil companies to evacuate their staff in the Niger Delta following three days of heavy clashes with the military.

The army said its forces, using navy gunboats and helicopters, would continue an offensive to flush militants out of the creeks after the hijacking of two oil vessels and attacks on troops in southern Delta state.

Global crude oil markets have largely ignored the clashes in the OPEC member country, closing lower in two of the last three sessions.

"We will carry on with our operations until we stabilize the situation," said Colonel Rabe Abubakar, spokesman for the military taskforce in the Niger Delta, adding two soldiers were wounded in this week's fighting.

Royal Dutch Shell, U.S. oil major Chevron and other energy companies working in the area were given until Saturday by the militants to remove their workers.

Security sources have said they are taking the militant threats seriously but there are no plans to evacuate more staff.

The military on Friday freed nine Filipinos and four Nigerians that were kidnapped two days earlier when their oil vessel MV Spirit was hijacked by militants near Warri in Delta state, Abubakar said.

A total of 20 people were believed to have been on board the ship chartered by state oil firm NNPC.

The rescued crew members told reporters on Saturday two Filipinos were killed and at least five others wounded by crossfire.

"We thought we were going to die as bombs were coming down from the helicopters, but the rescuers told us not to run," Mathillo Ete, a Filipino crew member wounded in the clashes, told Reuters.