(Adds new kidnapping in paragraphs 6-7)

By Tom Ashby

LAGOS, June 8 (Reuters) - Britain on Friday advised all its nationals to leave three states at the heart of Nigeria's southern Niger Delta, Africa's top oil-producing region, where violence against foreigners has become commonplace.

The advice came as the Supreme Court refused bail for a former militia leader whose detention has fuelled the unrest, raising pressure on newly inaugurated President Umaru Yar'Adua to address the causes of a crisis which has crippled exports.

Kidnappings, armed robberies and other attacks have risen in Bayelsa, Delta and Rivers states -- home to the bulk of Nigerian oil output and the local arm of Royal Dutch Shell <RDSa.L> -- and these are unsafe for Britons, the Foreign Office said.

"We therefore advise British nationals to leave. If you stay, you do so at your own risk," it said on its Web site.

Since January 2006, more than 180 foreign nationals have been abducted in the Niger Delta, including 31 British nationals, it added. Most have been released unharmed, often after the payment of a ransom.

On Friday, gunmen kidnapped a Lebanese manager in the Rivers state capital Port Harcourt, a security source said. The abduction brought to 31 the number of foreign hostages being held in different parts of the delta.

Police confirmed a manager from shipyard Modant Marine had been kidnapped but could not confirm the man's nationality.

A senior British oil executive said the Foreign Office advice was unlikely to trigger an exodus from the wetlands region because thousands of foreign workers had already quit and most of those left were already aware of the risks.

ANARCHY

Militants fighting for more autonomy in the delta began a wave of attacks on oil facilities and kidnappings early last year, but the line between militancy and crime is blurred and diplomats say the region is slipping into anarchy.

Attacks have cut Nigerian oil output by 772,000 barrels a day, or a quarter of the OPEC member nation's total capacity.

Militants demand local control over oil revenues and compensation for oil spills, reflecting popular sentiment in the impoverished delta where many residents feel neglected.

The rebels also want the release of Mujahid Dokubo-Asari, a former militia leader on trial for treason. The Supreme Court refused bail for Asari on Friday, saying the ethnic Ijaw nationalist was a threat to security.

Defence lawyer Festus Keyamo said the continued detention of Asari, who has already spent 20 months behind bars while his trial is repeatedly adjourned without tackling any substantive issue, would only prolong the crisis.

"We think the issues are bigger than Asari. He is only a symbol of the struggle," Keyamo said after the court ruling.

Yar'Adua used his inauguration address on May 29 to promise urgent attention to the delta and to call for a ceasefire.

"We want to tell Yar'Adua that there will be no solution to the violence in the Niger Delta unless he resolves the trial of Asari," Keyamo said.

Ijaw leaders and militants have been holding closed-door meetings with the new government over the past few days, and some expect a political solution to Asari's detention.

"There are discussions going on to find a political solution," said Oronto Douglas, a delta activist.

"The case of Asari and other political detainees will have to be tackled because it is a cardinal demand of the militants." (Additional reporting by Estelle Shirbon in Abuja)

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