Maltese officials have been tracking the Maltese-registered, Finnish-chartered ship.

"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 "Arctic Sea," as the vessel is called, has a crew of 15 Russians, and Russian President Dmitry Medvedev ordered his navy to join in the search in the Atlantic.

Security and maritime experts are at a loss as to exactly what has happened to the missing ship.

The ship left Finland on July 23 with a 915,000 euro ($1.3 million) load of lumber. It was supposed to dock in the Algerian port of Bejaia on August 4. But the ship reported it had been attacked in the waters off Sweden, although Swedish police said the attackers reportedly left after 12 hours.

It then sailed through the busy English Channel between France and Britain late last month.

Britain's Maritime and Coastguard Agency did make contact with someone on board the Arctic Sea as it passed through the Channel of July 24.

"There didn't seem anything suspicious when the contact was made. It could well be that a crew member had a gun put his to head by a hijacker when contact was made, but who knows?" said MCA spokesman Mark Clark.

He said attacks in European waters were extremely rare.

Experts have bandied about several theories for the ship's strange behaviour. While piracy could be involved, no ransom has been demanded. This could indicate a commercial dispute or that the ship has somehow become involved with the Russian mafia, according to some experts. Or the taking of the ship might simply be a test of European security.

"It is alarming that, in the 21st century, a ship can apparently be commandeered by hijackers and sail through the world's busiest waterway with no alarm being raised and no naval vessel going to intercept it," said Mark Dickinson, the general secretary of Nautilus International, a seafarers' union.

hf/AFP/AP/Reuters
Editor: Chuck Penfold