The Arctic Sea is thought to have headed out into the Atlantic Ocean
Russian navy ships are hunting in the Atlantic for a 4,000-tonne cargo ship that vanished more than two weeks ago, triggering an international mystery.
Observers have suggested the ship was hijacked, possibly because of a Russian commercial dispute.
The Maltese-flagged ship, the Arctic Sea, has a 15-strong Russian crew.
Carrying timber reportedly worth $1.8m (£1.1m), it sailed from Finland and had been scheduled to dock in the Algerian port of Bejaia on 4 August.
The ship was last sighted in the Bay of Biscay on 30 July, according to the Russian media.
There were reports that the ship may have been spotted subsequently by a Portuguese coastal patrol aircraft, but a navy spokesman in Portugal said it had not passed through Portuguese waters.
Earlier in its voyage, the ship reported being boarded by up to 10 armed men as it sailed through the Baltic Sea on 24 July, but said the intruders left the vessel on an inflatable boat after 12 hours.
Malta's maritime authority said it had "not approached the Straits of Gibraltar, which indicates that the ship headed out in the Atlantic Ocean".
Russian navy chief Adm Vladimir Vysotsky said there were five Russian warships and other vessels searching in the Atlantic.
"We have no concrete data on the location of the vessel as of the moment," he told Russian news agency Itar-Tass.
"All the information-intelligence systems of our armed forces, including the space-borne systems, the system of identification of sea vessels and others, have been calibrated to look for the missing ship," he added.
A Russian diplomat said that Russia was "actively working through diplomatic channels with all interested sides".
Analysts have said that the ship is likely to have been seized, but have stressed that it is a highly unusual case that appears to have occurred in an area where there are not normally pirate attacks.
David Osler, who writes on maritime safety for the newspaper Lloyds List, said Swedish police had been investigating whether the disappearance of the ship was the result of a dispute between the ship's owner and another party.
"The longer it goes on, the more it looks like some sort of dispute between Russian interests," he told the BBC.
Mikhail Voitenko, editor of Russia's Sovfracht maritime bulletin, said the ship may have been boarded because it was carrying a valuable, unknown cargo.
"We have to remember that before loading in Finland the vessel stayed for two weeks in a shipyard in Kaliningrad," he told the Russia Today TV channel.
"It seems some third party didn't want this transit to be fulfilled so they made this situation highly sophisticated and very complicated."
But the head of the ship's operator, Nikolai Karpenkov, dismissed speculation that there might be a mystery cargo on board.
He told Itar-Tass that the ship had been checked when leaving Kaliningrad after repairs, and in Finland, where he said nothing illegal or suspicious had been found.