UPDATE 1-Nigerian army frees hostages, destroys militant camp

* Security forces rescue 13 hostages, including 9 foreigners

* Destroy important militant camp in delta

(Adds comments from rescued hostages, previous Port Harcourt)

By Segun Owen

WARRI, Nigeria, May 16 (Reuters) - 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 CLc1 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.