* Russian sailors arrested in Nigerian waters in October
* Nigerian authorities say seized thousands of munitions
* Russia says sailors held under "false pretext" (Adds comment from Russian foreign ministry)
ABUJA, Jan 9 (Reuters) - Fifteen Russian sailors detained by Nigeria's navy in October on suspicion of arms smuggling have been transferred to police cells for prosecution, a naval spokesman said on Wednesday.
Nigerian authorities intercepted a ship and arrested its Russian crew on Oct. 23 after they found several guns and around 8,500 rounds of ammunition aboard.
"On Monday, we handed over the crew to the police as we have completed preliminary investigations," Lieutenant Commander Jerry Omodara, spokesman for the Western Naval Command, said by telephone. "The ship and the weapons are still in our custody."
The Russian Foreign Ministry said the sailors were facing a court case initiated "under false pretexts" and that Nigeria broke a promise given to Foreign Minister Sergei Lavrov that the men would return to Russia before the New Year.
"We expect that this time around the Nigerian partners will stick to their word and release the Russian sailors in the nearest time without additional conditions and further delays," the ministry said in a statement on Wednesday.
Arms smuggling to and through Nigeria is rife. Demand for weapons is great because of an Islamist rebellion in the north, armed robbery and kidnapping by gangs in the south and oil theft and piracy in the southeast.
The country is also sometimes used as a conduit for shipping arms to other conflict-ridden parts of West Africa.
The Moran Security Group, the Russian company that owns the confiscated vessel, called the accusations "groundless" and urged Nigeria to release the sailors.
"Over nearly three months, no charges were presented. This means the Nigerian side has no strong evidence... A week after the detention they would have been transferred to police arrest and then to court," Moran spokesman Alexei Maximov said.
"This case ... is the worst possible scenario that is threatening bilateral ties between Russia and Nigeria," he said.
Omodara said the boat was stopped initially because it was in Nigerian waters without permission.
In 2010, a consignment of rocket launchers, grenades and other explosives from Iran was seized in Lagos, causing a diplomatic row between Nigeria and Iran. It also strained ties between Iran and Senegal, which accused Iranian security forces of trying to supply weapons to its Casamance rebels. (Reporting by Isaac Abrak; Additional reporting by Gabriela Baczynska in Moscow; Writing by Tim Cocks; Editing by Tom Pfeiffer)