Armed men attacked a convoy in Nigeria's oil-rich southeastern delta Wednesday, sparking a gunbattle with security forces, a government security official said.

Brig. Gen. Alfred Ilogho said the convoy was carrying soldiers and supplies for a Agip, a subsidiary of Italian oil company ENI SpA. He said he had no details on casualties.

ENI could not immediately be reached for comment.

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

Shortly after the attack was reported, an Associated Press reporter in Port Harcourt saw several attack helicopters leaving an air force base and heading west, toward the reported site of the attack.

Ilogho declined to comment on their destination, saying they were not under his command. Military leaders in the eastern delta were unavailable for comment.

Earlier Wednesday, militants released the last 16 Nigerian hostages, two days after the oil workers were seized, Royal Dutch Shell PLC said. Several foreign hostages taken in another attack on Tuesday night remained in captivity.

Biji Ojediran, a spokesman for the oil consortium led by Royal Dutch Shell, said that the last of the 25 hostage seized in a deadly raid Monday had been liberated overnight after nine had been set free earlier. At least five soldiers were killed in the attack Monday, with nine more still missing.

On Tuesday, gunmen killed two security guards before seizing several foreign oil workers, according to security officials. The British Embassy confirmed four Britons were believed among those taken outside Port Harcourt, the main oil-industry hub.

State police spokesman Suur Chafa said a Romanian, an Indonesian and a Malaysian were also missing and believed taken hostage.

Attacks in the region in the past year have cut Nigeria's oil output by almost 25 percent. Nigeria is Africa's biggest petroleum producer and one of the main suppliers of crude oil to the United States.

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.

Despite the Niger delta's massive energy resources, the vast majority of the region's people have no access to clean water or electricity and live in extreme poverty.

Militants in the area say they are fighting for their rightful share of oil proceeds, which are paid directly to the federal government. Kidnappings and attacks that grab international attention, some militants say, are the only means of protest available to them.