LAGOS, Nigeria - - A major pipeline feeding one of Royal Dutch Shell PLC's two main oil export terminals in southern Nigeria was attacked and ruptured by unknown assailants, the company said Thursday.

The pipeline supplies crude oil to the Forcados oil export terminal, said Precious Okolobo, a spokesman for Shell in Lagos. He would not give further details on the nature of the attack or when it was carried out.

Nigeria is Africa's leading oil exporter and the fifth-biggest source of U.S. oil imports.

Oil exports through Forcados have suffered in the past two years as attacks by armed groups in the main oil-producing Niger Delta region forced shut many fields supplying the terminal, which can handle 400,000 barrels of crude daily.

A lull following the emergence of President Umaru Yar'Adua in May and a cease-fire called by the main militant group, Movement for the Emancipation of the Niger Delta, or MEND, enabled Shell to restart production in some of the abandoned fields.

Attacks by MEND and other armed groups in the region have cut Nigeria's oil exports of 2.5 million barrels daily by more than 20 per cent in the last two years.

MEND, which claims to be fighting for a greater share of oil wealth for the delta's impoverished inhabitants, ended its cease-fire after one of its leaders, Henry Okah, was arrested in Angola in September on gunrunning charges the group said were instigated by Nigeria.

The group said Wednesday it was embarking on a violent new phase. It claimed responsibility for an attack on an ExxonMobil facility Monday, but denied it was behind another raid that day in a nearby border area in which 21 troops from Cameroon were killed.

The attack on a military position outside ExxonMobil's Qua Iboe oil export terminal was aimed at seizing weapons and equipment, and was a success, MEND said in an e-mail sent Wednesday night.

The group alleged Nigerian troops had attacked the Cameroonian positions in formerly disputed Bakassi peninsula "because of their perceived sympathy to our cause and their blind eye to a weapons route."

Nigeria denies the allegation, and has in turn blamed the militants for the attack.