Nigerian troops battled militia fighters in swamps around a Royal Dutch Shell PLC oil platform that militants attacked at dawn Sunday, the third assault on Shell oil facilities in less than a week in the troubled region.

Shell confirmed the attack on the Benisede oil platform in the southern oil-rich Niger Delta and said some of its staff had been injured and taken to hospital. The company also said it had begun evacuating personnel from vulnerable facilities in the region due to worsening security.

In a statement, Shell said "heavily armed persons" in speed boats attacked the platform early Sunday, burning down staff accommodations and damaging the facility.

Soldiers guarding Benisede returned automatic weapons fire, but it was unclear if they had lost control of the oil platform, said Brig. Gen. Elias Zamani, commander of a special task force charged with security in the volatile oil region.

Zamani had no other details of the fighting and said the military was investigating.

Residents of the area reported continuous gunfire and the explosion of heavy guns for most of Sunday after troop reinforcements moved in.

"There is a war going on here," Enitowari Inengi, a resident of the Ozobo fishing community near Benisede, said by telephone. "People are scared and are taking their boats and moving away."

Another resident, Nelson Wariebi, said he had seen military helicopters moving in to attack positions held by the militants.

On Wednesday, gunmen attacked Shell's EA platform in shallow waters near the delta coast, seizing a Bulgarian, an American, a British and a Honduran. A major Shell pipeline leading to its Forcados export terminal was blown up the following day.

Though Shell resumed some production cut last week, the first two attacks initially forced a 10 percent drop in Nigeria's oil exports.

A previously unknown militant group, Movement for Niger Delta Emancipation, claimed responsibility for first two attacks, warning all Western oil companies to leave the Niger Delta for their safety and calling on the government to release militia leader Mujahid Dokubo-Asari.

Dokubo-Asari campaigned for secession and greater local control of oil wealth before he was jailed in September and charged with treason.

Nigeria is Africa's leading oil exporter and the fifth-biggest source of U.S. oil imports. The country produces about 2.5 million barrels a day.

Violence, hostage-taking and sabotage of oil operations have been common in the oil-rich Niger Delta in the past 15 years amid demands by the region's impoverished communities for a greater share of the oil revenue flowing from their land.