The Arctic Sea was reportedly boarded by up to 10 gunmen
Russia's navy has been deployed to find a ship reportedly hijacked three weeks ago in the Baltic Sea.
Up to five vessels - reported to include nuclear submarines - will be involved in the search for the Maltese-flagged Arctic Sea, the navy confirmed.
It has a 15-strong Russian crew and was reportedly taking timber worth $1.5m (£900,000) from Finland to Algeria when it was boarded by gunmen on 24 July.
The Arctic Sea was last sighted off the north coast of France on 30 July.
British authorities say the 4,000-tonne vessel may have been spotted subsequently by a Portuguese coastal patrol aircraft, but its current location remains unknown.
Maltese authorities have said it is unlikely to be in the Mediterranean.
"It would appear that the ship has not approached the Straits of Gibraltar, which indicates that the ship is headed out into the Atlantic Ocean," the Malta Maritime Authority said in a statement.
The Portuguese Navy too has said that the missing cargo ship has not passed through Portuguese waters.
Russian naval commander Adm Vladimir Vysotsky told Itar-Tass news agency that all Russian navy ships in the Atlantic had joined the search for the vessel.
Operations will be centred on the patrol ship Ladny, which is part of Russia's Black Sea fleet.
UK authorities, which made contact with the Arctic Sea before it entered the busy shipping waters of the English channel, described the situation as "bizarre".
"Who would think that a hijacked ship could pass through one of the most policed and concentrated waters in the world?" said Mark Clark of the UK's Maritime and Coastguard Agency (MCA).
"There didn't seem anything suspicious when contact was made," he added. "It could well be that a crew member had a gun put to his head by a hijacker when contact was made."
The Finnish shipping line operating the ship reportedly said it was boarded by up to 10 armed men claiming to be anti-drugs police as it sailed through the Baltic Sea on 24 July.
But the intruders are reported to have left the vessel 12 hours later on an inflatable boat, and it is unclear who is in current command of the ship.
The Arctic Sea had been scheduled to dock in the Algerian port of Bejaia on 4 August.
While world leaders have become increasingly concerned about pirates operating off the coast of Somalia, maritime experts suggest the case of the Arctic Sea reflects a different kind of piracy.
Nick Davis, who runs the private security firm Anti-Piracy Maritime Security Solutions, told the BBC that the relatively low value of the cargo suggested the ship's seizure may be the result of a "commercial dispute" in which one party had decided to "take matters into their own hands".
But he added: "Piracy is piracy - if someone's wanting to take that vessel, and they're not authorised, and they use a speedboat to go and get it, then it's no different to what the Somalis do."
Relatives of the Arctic Sea's 15 crew members - all of whom are said to come from the northern Russian port city of Arkhangelsk - have so far been unwilling to speak to the media.