By Nick Tattersall

LAGOS, July 3 (Reuters) - The suspected leader of Nigeria's main oil militant group has won the right to appeal to have his trial for treason and gun-running held in public, a move which could placate his well-armed supporters in the Niger Delta.

Henry Okah is one of the senior leaders of the Movement for the Emancipation of the Niger Delta (MEND), a militant group which has cut Nigeria's oil output by around a fifth since early 2006 with a campaign of attacks on the oil industry.

Okah still commands loyalty from several armed factions in the delta and Nigeria has said his trial at a federal court in the central city of Jos must be held in camera for the sake of national security. But the secrecy has angered his supporters.

Okah's lawyer, Wilson Ajuwa, said an appeals court had accepted his bid to have the case heard in public. He said the court had told both parties to file written briefs within 21 days and that it would consider the appeal in September.

"Once we file the briefs and we are able to convince the appeals court then the federal court will have to stop the secret trial," Ajuwa told Reuters on Thursday.

The trial, which is due to resume on Monday, would continue in camera until the appeals court ruled, he said. Okah could face the death penalty if convicted.

CAMPAIGN OF SABOTAGE

MEND has made Okah's release a condition for suspending its campaign of sabotage and for taking part in peace talks planned by the government of President Umaru Yar'Adua.

Oil pipeline bombings and attacks on industry installations in the delta have helped push world oil prices to record highs and Yar'Adua is under immense pressure to bring stability.

MEND carried out its most spectacular sabotage to date last month, attacking Royal Dutch Shell's Bonga field some 120 km (75 miles) from the shore and forcing the Anglo-Dutch giant to shut down its near 220,000 barrels per day production.

That strike effectively cancelled out the impact of an agreement a few days later by Saudi Arabia to hike production by roughly the same amount, a bid to assuage the burden on consumer countries struggling to cope with higher fuel costs.

Yar'Adua has said his administration will take a two-pronged approach to the delta, pledging development for communities whose land and water has been polluted by oil extraction but also saying he will not tolerate the presence of armed groups.

He said in an interview on state television in May, marking his first year in power, that Okah must be tried in secret.

"Okah's trial has implications for national security. That is why our security agencies advise and insist that for national security reasons he should be tried in camera," Yar'Adua said.

"You cannot jeopardise national security for any reason."