A ransom demand has been made of the owners of the missing cargo ship believed to have sailed through the English Channel after being boarded by pirates, it was disclosed yesterday.

The Finnish, Swedish and Maltese authorities are investigating offences of aggravated extortion and alleged hijacking in relation to the Maltese-flagged Arctic Sea, which disappeared with a 15-strong Russian crew on board more than two weeks ago.

Experts and maritime authorities have been left baffled since the ship's disappearance, which sparked international search efforts. A statement from the Finnish police said: "The Finnish, Swedish and Maltese authorities have conducted an investigation in close cooperation into the alleged offences relating to the cargo vessel Arctic Sea.

"The investigated offences are aggravated extortion and alleged hijacking."

The ship's current location remains unclear but there were unconfirmed sightings of it about 520 miles off the Cape Verde islands off the west coast of Africa yesterday.

The vessel's tracking system was reported to be broadcasting signals from the Bay of Biscay off France yesterday.

Russian maritime website Sovfrakht said the signal appeared on a tracking service at around 8.30am and lasted for an hour but added that it was not known if the AISLive ship tracking system equipment was still actually on the ship.

The 4,000-tonne vessel was meant to arrive in Bejaia in northern Algeria with its cargo of around £1m worth of sawn timber on 4 August. Its last official recorded positioning was off northern France on 30 July.

The ship's last known radio contact was with British coastguards when it made routine contact with Dover Coastguard as it was about to enter the Strait of Dover from the North Sea at 1.52pm on 28 July.

Days later Interpol informed the British Coastguard the ship had been hijacked days before in the Baltic Sea.

According to reports, it was boarded by up to 10 armed men purporting to be anti-drugs police on 24 July.