Nigeria says after three days of heavy clashes with militants, its security forces have rescued 13 hostages and destroyed a key militant camp.

The military said they were successful in rescuing nine Filipinos and four Nigerians, out of a total of 20 people, who were kidnapped two days earlier when their oil vessel MV Spirit, chartered by oil firm NNPC, was hijacked by militants near Warri in the heart of Africa's biggest oil industry region of Niger Delta.

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

The militant camp, located along Chanomi Creek in Delta state near Chevron's Escravos export terminal and Nigeria's 125,000 barrels per day Warri refinery, was largely destroyed on Friday in the heaviest fighting in the area in eight months, Colonel Rabe Abubakar, spokesman for the military taskforce in the Niger Delta said.

The heavy fighting has been centered around the camp belonging to militant leader, Government Tompolo, who had been involved in negotiations over a possible amnesty with the authorities.

As the clashes are continuing, his militant group has declared an "all-out war" and warned oil companies to evacuate their staff in the Niger Delta.

The army said the offensive will continue until the situation is stabilized.