Troops began a search-and-rescue mission Thursday for Nigerian soldiers missing after militants ambushed a military-escorted supply convoy in the restive, oil-rich south, officials said.

Brig. Gen. Alfred Ilhogo confirmed the attack late Wednesday on the convoy bringing provisions to Agip, a subsidiary of Italian oil company Eni SpA, in the Niger Delta region where much of the crude in Africa's largest producer is pumped.

The convoy was on its way to an export terminal about 50 miles west of Nigeria's oil capital, Port Harcourt, Ilogho said.

"I can't tell you if there have been any fatalities yet," he said of the attack. "There was an engagement and there were some soldiers missing."

A lower-ranking officer, who was not cleared to speak to the media and spoke on condition of anonymity, said that nine out of 11 soldiers escorting the convoy had failed to return to base.

Eni spokesman Gianni Di Giovanni said late Wednesday the company did not have information about the attack.

"Attacks are frequent, and so is their lack of success. We must wait," Di Giovanni said.

A series of attacks and kidnappings in Africa's largest oil producer have cut Nigeria's production by more than a quarter so far this year. More than two dozen oil workers have been kidnapped this year. Hostage takings generally end peacefully, with the targets returned unharmed.

Several foreigners kidnapped Tuesday night remained in captivity.

The hostage-takers, who haven't been identified, have demanded $10 million each for the captive foreigners, according to a Western diplomat who spoke on condition of anonymity citing prohibitions on dealings with the media.

Chief Sara Igbe said militants attacked a pumping station in his community of Ekulama, about 25 miles west of Port Harcourt, sparking a battle with government forces. The military said it had no confirmation and Shell said production wasn't hit.

An e-mail from the Movement for the Emancipation of the Niger Delta — a major militant group — said they killed nine soldiers and captured two gunboats in an hour and a half of fighting at the facility. The group, also known as MEND, said they acted defensively after being attacked by army gunboats and a helicopter gunship.

The militants said they planned further attacks and warned oil companies to evacuate staff in the region.

Witnesses said they saw government attack helicopters racing west from Port Harcourt toward the areas where the two attacks occurred.

Despite the Niger delta's massive energy resources, the vast majority of the region's people are mired in extreme poverty and many say kidnappings and attacks that grab international attention are some of the only tools available to them.

Groups have sabotaged pipelines and oil installations in what they say are protests against the international oil companies and the federal government, which controls the petroleum revenue and divvies it up among Nigeria's 36 states.

The West African country is the fifth-largest supplier of crude oil to the United States.