Junior naval officer, Solomon Attam, told the seven-member court martial the crude oil in the tanker, MT African Pride, was emptied and replaced with water.

Unnamed superior officers offered him and his three colleagues one million naira (7,500 dollars) to look the other way as the deed was happening, he said.

He was giving evidence in the case of three senior Nigerian navy officers who have been charged with conspiracy in the disappearance of the tanker which was impounded by the military last year.

A senior officer admitted to the panel that he failed to do his job of properly monitoring the ship before it sailed out of the dock and disappeared mysteriously.

Commodore Olukayode Biobaku, a naval senior intelligence officer, said Thursday that when he had learned that some people were planning to tamper with the ship, he told his boss who took immediate measures.

The trial formally began last November 1, and the accused, navy rear admirals Babatunde Kolawole, Francis Agbiti and Anthonio Bob-Manuel, have pleaded not guilty to the charges.

Every day, international smugglers carry off tens of thousands of barrels of crude -- siphoned from illegally tapped pipelines in the Niger Delta -- a trade which has financed an arms race among violent gangs in the oil-rich region.

Nigeria is the world's sixth biggest oil exporter, accounting for a daily output of around 2.5 million barrels.

The tanker was seized in October last year by the Nigerian navy along with its crew of 13 Russian sailors.

The crew is on trial in a civil court accused of attempting to smuggle 11,300 tonnes of crude oil worth 345 million naira (2.6 million dollars, two million euros) out of Nigeria.

The vessel was brought to Lagos in January and placed in the custody of security agents, pending the trial, but later disappeared.

A parliament committee investigating the case has described the mysterious disappearance of the ship as a "national embarrassment".

Officials say Nigeria, Africa's largest oil producer, loses some 100,000 barrels per day of crude oil to thieves.

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