(Adds trial in paragraphs 8-11)

By Austin Ekeinde

PORT HARCOURT, Nigeria, May 22 (Reuters) - Four gunmen seized a Lebanese oil executive in Nigeria's southern city of Warri on Tuesday, the army said, taking the number of foreign hostages in Africa's top oil producer to 15.

An 18-month upsurge of violence in Nigeria's southern swamps, home to all the country's oil reserves, has dented crude exports, hit investments and forced thousands of foreign workers to flee.

"A Lebanese was kidnapped by gunmen at the Enerhen junction this morning. We are waiting to get details of those responsible for the kidnap," Brigadier-General Lawrence Ngubane, commander of the military task force in Warri, told Reuters.

The attackers also snatched the Lebanese man's car and a gun was recovered at the scene. The man was a financial controller at local oil service company Nigercat, security sources said.

The violence is linked to an insurgency by militants fighting for more autonomy in the Niger Delta, but the line between militancy and crime is blurred.

More than 100 foreigners have been taken hostage in the region since January, but most of them were freed after their employers paid ransoms.

Militants have demanded regional control over the delta's oil wealth, compensation for decades of oil spills, and freedom for two jailed leaders from the region.

One of them, Mujahid Dokubo-Asari, appeared in court on Tuesday on treason charges. The prosecution asked the judge to hear the case in private, saying the identity of witnesses from the security agencies would be jeopardised by a public trial.

Asari's lawyer said the prosecution was using delaying tactics. The trial has been in court for 20 months and neither side has presented a substantive argument.

"The trial is just a smokescreen to keep him out of circulation," Festus Keyamo told Reuters.

"It is not meant to try him for any offence he has committed. They still don't know how to handle the Niger Delta crisis. The level of violence has caught them unawares."

Currently about one quarter of Nigeria's total oil production capacity is shut due to insecurity in the delta, lifting world oil prices at a time when Nigeria's high quality oil is in particularly strong demand.

On Saturday, ransom-seekers attacked a residential compound with dynamite and machine guns in the delta city of Port Harcourt and took two Indian petrochemical workers.

The Indian government said contact has been established with the abductors and that the hostages were safe and their release was imminent, Indian media reported.

Officials said the negotiations were conducted by private parties and they expect a positive outcome within the next 24 hours. (Additional reporting by Tume Ahemba in Lagos; Tom Ashby and Estelle Shirbon in Abuja)

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