A Nigerian court dismissed charges against eight of 15 Russian seamen arrested in October for alleged illegal arms possession, but set a trial date for later this week for the others, Lagos.
The vessel, the MV Myre Seadiver, was seized in October by a naval patrol which also detained its 15 crew members off the coast of Lagos.
The owner of the vessel is Moscow-based Moran Group, a private security firm.
Among weapons found on the vessel there were 14 AK-47 rifles with 3,643 rounds of ammunition, 22 Benelli MR1 rifles with 4,955 rounds of ammunition.
The defender has argued that the suspects were innocent and the ship was contracted to provide security to vessels in the region, which sees repeated pirate attacks. Also permission was sought and secured before the vessel entered Nigeria's territorial waters.
"The charges against eight of the accused are hereby struck out following an application by the prosecution to withdraw the charges against them," federal high court judge James Tsoho ruled during a hearing in Lagos.
The trial of the remaining seven suspects will begin on Friday. Precise details of the decision to release eight of the suspects were not made clear.
Defence lawyer Abubakar Unegbu said the charges against the eight were dropped because there was "overwhelming evidence" that they came to Nigeria legally.
"The eight are innocent of the charges of illegal entry into the country. They were given visas to enter Nigeria by the Nigerian embassy," he said.
However, it was unclear why they would no longer face charges of illegal arms possession. The suspects were released on bail in February following a written commitment by the Russian ambassador in Nigeria to produce them on demand.
Then the court ordered the vessel be released after the payment of a bond in the sum of $500,000.
In December, Russia's foreign ministry said it had reached a deal to secure the release of the seamen, explaining that Nigeria had agreed to drop the case to preserve cordial relations between the two nations. That pact, if it ever existed, appears to have been sidelined.