ABUJA, Feb 29 (Reuters) - A Nigerian rebel group from the oil producing Niger Delta accused the government on Friday of denying their detained leader access to lawyers and relatives despite a court order that he should be allowed to see them.

Henry Okah of the Movement for the Emancipation of the Niger Delta (MEND) was arrested in Angola on Sept. 3 and handed over to Nigeria on Feb. 14. His detention in a secret location by Nigerian authorities has raised tensions in the delta.

"The Nigerian government has blatantly refused to comply with an Abuja High Court order granting the lawyers and families of Henry Okah and Edward Atatah immediate access to them," the MEND said in an email to journalists, threatening "anarchy".

Atatah was with Okah in Angola and was arrested and later transferred to Nigeria with him. Lawyers for the two men have said they were in Angola to see a trawler they wanted to buy.

Government spokesmen could not immediately be reached. They have consistently declined to comment on Okah since his return.

The MEND first emerged in late 2005 and in early 2006 it blew up several major oil production facilities and kidnapped dozens of foreign oil workers, forcing the closure of a fifth of Nigeria's crude output and pushing up world oil prices.

Like other armed groups in the impoverished delta, it says it is fighting to redress injustice. Five decades of oil extraction in the delta have enriched corrupt politicians and foreign oil firms, while communities in the vast maze of creeks have seen their water and land polluted and have no electricity.

But militancy and crime are intertwined in the delta, where armed groups can make big profits from taking hostages for ransom or tapping into pipelines to siphon off crude oil.

Okah has yet to be charged with any crime, although the police last week issued a press statement accusing him of killings, arms dealing, crude oil smuggling, financing militant attacks, sabotage of the oil industry, piracy and secessionism.

The Okah case is reminiscent of the arrest in September 2005 of another prominent Niger Delta militia leader, Mujahid Dokubo-Asari. His detention and trial for treason were one of the factors that spawned the MEND and the 2006 crisis.

Asari was freed in June 2007, a political decision taken a few days after new President Umaru Yar'Adua took office. Yar'Adua and his deputy, who is from the delta, have tried to negotiate with the rebels but Okah's detention is a setback.

Several militant leaders have already signalled they may pull out of the talks, partly because the government's failure to produce Okah in public has fuelled rumours that he may have been tortured or injured in detention. Police have denied this. (Reporting by Estelle Shirbon; Editing by Matthew Tostevin)

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