The MT African Pride was seized in October last year by the Nigerian navy along with 13 Russian sailors.

The crew is on trial accused of attempting to smuggle 11,300 tonnes of crude oil worth 345 million naira (2.6 million dollars / two million euros) from waters off the coast of Warri at the heart of the oil-rich but troubled Niger Delta.

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

"Some officers have appeared before a board of inquiry over the incident. They are being questioned over their role in the matter," navy spokesman Sinefi Hungiapuko told AFP. Press reports said eight officers were facing the probe.

Hungiapuko said the findings of the board will determine if and when a general court martial will try any officer found culpable in the loss of the vessel.

"We will not hesitate to punish any officer connected with the disappearance of the vessel," he added.

Nigeria's House of Representatives is currently probing the missing Russian tanker, whose loss has been described as national embarrassment.

Another vessel 'MT Jimoh' which was discovered to also be missing, was found last Tuesday near the southern oil city of Port Harcourt.

But the case of the Russian vessel may affect the ongoing trial of the 13 Russians in a federal high court in Lagos.

The trial resumed last week after a long adjournment, with no mention of the vessel during proceedings. The case has been adjourned to October 19 for further hearing.

Earlier in the trial in May, defence counsel Emefo Etudo alerted the court that he had information that some navy officers "had tampered with the cargo", but the presiding judge, Gloria Okeke, dismissed the claim.

Officials say Nigeria, Africa's largest producer, loses some 50,000 barrels per day of crude oil to thieves.

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

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