Intervene in Nigeria, Bush urged

In their open letter, the militants pointed out that the US receives a large proportion of its crude oil from the Niger Delta.

President Bush is on a tour of Africa but Nigeria is not on his itinerary.

Attacks by militants have drastically cut Nigeria's production and helped drive up the price of oil.

The appeal came from a faction of the Movement for the Emancipation of the Niger Delta (Mend) loyal to Henry Okah.

Mr Okah was extradited from Angola to Nigeria last week. His exact whereabouts are not known.

The BBC's Alex Last in Lagos says Nigeria's militants have used Mr Bush's trip as an opportunity to attract publicity.

Last month they tried to invite the Hollywood actor George Clooney to intervene in the crisis.

Criminal activities

The militant faction is demanding the release of Mr Okah following his extradition.

Mend claimed he was being held at a secret location in northern Nigeria, but ministers in Abuja have not commented.

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Mr Okah is a founder of Mend. He was arrested in Angola five months ago on arms trafficking charges.

The group came to prominence in the Delta two years ago when it cut Nigerian oil production by a quarter.

While other militant groups have called a ceasefire after being offered government incentives, Mr Okah's faction has remained active.

They have carried out a series of attacks in the region since his arrest last September.

Earlier this month, Nigeria's government told all oil companies which had fled attacks in the Niger Delta to return or stop working, saying the area was now safe.

Nigeria is Africa's leading oil producer but more than half of its people live in poverty.

Our correspondent says although the people of the Delta have genuine grievances, many of the armed groups are thought to be involved in criminal activities.

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