A prominent militant group in Nigeria's oil producing south says a major attack on the oil industry is imminent and has warned foreigners to leave the region for their own safety. Gilbert da Costa reports the threat comes days after militants attacked four ships in the troubled region.

The Movement for the Emancipation of the Niger Delta says the planned attack will be of such magnitude that it will cause what it describes as an "economic tsunami" in the world oil market.

MEND has claimed responsibility for a string of kidnappings of foreign oil workers as well as attacks on oil facilities in the past two years.

Military and intelligence sources acknowledge oil rebels are stockpiling weapons and supplies in preparation for a big offensive.

Army spokesman in the main oil city, Port Harcourt, Major Sagir Musa, says the Nigerian military is taking the threat very seriously.

"We are always, especially with these threats, we are on the alert and there is serious supervision, serious reinforcement, serious fortification of our positions and our guard locations," said Musa. "And, all the trouble spots are being monitored, seriously, by relevant security agencies."

Nigeria's most powerful rebel force, which claims to be seeking a greater share of the oil revenues for impoverished Delta residents, also admitted, Thursday, it attacked four oil ships in the delta, earlier this week.

Major Musa says the military repulsed the rebel ambush.

"Yesterday, there was an attempted attack on some boats along Bonny River. And, the maritime component of the Joint Task Force Operation Flush Out 3 in Rivers state successfully repealed the attack," added Musa. "The attack was a failure, as the militants, or attackers, could not achieve their mission."

The Nigerian military has bombarded suspected rebel strongholds, in recent days, stoking tensions in the Delta creeks.

Attacks by militants in the unruly region have cut oil production by 25 percent, since the start of 2006. Nigeria is Africa's biggest crude producer and eighth largest world exporter. The West African country is capable of producing around 2.6 million barrels per day.