The ship's disappearance continues to puzzle experts
Russia has denied media reports that a cargo ship which was apparently hijacked in July was carrying Russian S-300 anti-aircraft missiles to Iran.
Foreign Minister Sergei Lavrov said the reports that the ship had illegal arms on board were "absolutely untrue".
The Maltese-flagged Arctic Sea vessel with 15 Russian crew went missing for more than two weeks. It was found on 16 August off West Africa.
Eight men were later charged with hijacking and piracy over the case.
The men, mostly from Estonia, are suspected of seizing the ship and its crew after raiding it disguised as police.
Speaking in Moscow, Mr Lavrov dismissed media speculation about S-300 missiles on board the Arctic Sea as "groundless".
Russia's top diplomat also promised a "transparent" investigation in which Maltese officials would also be invited to take part.
The 4,000-tonne vessel vanished in July days after leaving Finland with an apparent cargo of timber worth $1.8m (£1.1m), destined for the Algerian port of Bejaia.
Last week, Britain's Sunday Times quotes sources in Russia and Israel claiming that the Arctic Sea was carrying arms to Iran and not timber.
It said that the sources claimed the ship had been loaded with S-300 missiles, Russia's most advanced anti-aircraft weapon, while undergoing repairs in the Russian port of Kaliningrad.
The arms were sold by former military officers linked to the underworld, the Sunday Times reported.
Also last week, a Russian journalist fled his country after suggesting that the ship might have been carrying illegal weapons.
Mikhail Voitenko, the editor of the Sovfracht online maritime journal, said he had been told to leave Moscow or face arrest.
Speaking to the BBC from Turkey, Mr Voitenko said he had received a threatening phone call from "serious people" whom he suggested may have been members of Russia's intelligence agency, the FSB.
The FSB has made no public comments on the allegations.
There has also been speculation the ship may have been intercepted by Mossad - Israel's foreign intelligence service - in order to prevent a shipment of illegal arms to the Middle East.