The Nigerian military has confirmed the hijacking of a vessel in the Niger Delta and the kidnapping of 15 foreigners on board. 

The army said the ship, an oil tanker known as MV SPIRIT, was contracted by state-owned Nigerian National Petroleum Company to transport a shipment to Warri Port, in the Niger Delta.

Another ship, a cargo vessel, was also seized by militants, the army said.  A Nigerian newspaper reported at least 20 foreigners were abducted in the two incidents in Delta state.

The region's most prominent armed group, the Movement for the Emancipation of the Niger Delta, on Wednesday ordered oil companies in the Niger Delta to evacuate their staff within 72 hours or risk being caught up in "an emerging civil war."

The ultimatum followed heavy clashes with government forces in the southern Delta state this week.

The government is considering granting conditional amnesty to the militants.  A journalist in the delta region, Cletus Opukeme, says the new campaign of violence in the Niger Delta may have, in part, been induced by frustration over lack of progress in bringing development to the region.

"Instead of amnesty, the need development, they need massive development of the Niger Delta; development of capital projects, they need bridges, they need buildings, they need higher institutions," he said. "Amnesty for them simply means the Niger Delta struggle should go into oblivion."

Attacks by militants on oil facilities in the Niger Delta, home to Africa's biggest oil and gas industry, have shut down around a fifth of Nigerian output since early 2006.  Nigeria currently pumps less than two million barrels per day.

Officials say millions of dollars are lost daily in crude oil stolen in the Niger Delta.  The gunmen also regularly carry out kidnappings and robberies.

The latest attacks threaten fresh fighting in the strife-wracked Niger Delta where militants launched an oil war last year.

MEND is a loose coalition of militant groups who say they are fighting for a greater control of the region's oil wealth after decades of neglect.