Pirates probably hijacked a merchant ship which disappeared after sailing through the English Channel last month, its operator said.

The Kremlin has ordered Russian warships to join the hunt for the 4,000-tonne, 98-metre bulk carrier Arctic Sea, whose mysterious fate has baffled national maritime authorities across Europe and North Africa.

The Maltese-registered vessel, carrying a cargo of timber worth $1.3 million, was supposed to have docked on August 4 in the Algerian port of Bejaia.

It never arrived, raising fears of a rare case of piracy in northern European seas.

'My view is that it is most likely that the vessel has been hijacked,' Viktor Matveyev, director of the Finnish company Solchart, which operates the vessel, told Reuters. 'It is unclear where the vessel is now.'

A wave of piracy has hit shipping off Somalia, and an international naval force patrols its coast in an effort to protect merchant vessels. But a hijacking in European waters would be almost unprecedented in modern times.

'If this is piracy, and it seems most likely of all that it is, then it is one of the first cases in recent history of piracy in these seas,' said Matveyev.

Concerns over the safety of the 15-member Russian crew were raised after the Malta Maritime Authority said it received reports the ship had been boarded by armed men in masks posing as anti-drugs police in Swedish waters on July 24.

Swedish authorities said none of its law enforcement agencies had been involved.

Crew members were assaulted, tied, gagged and blindfolded and some were seriously hurt, Malta said.

'We have no details about the crew though we of course hope that everything is okay with them,' said Matveyev. 'We very much hope the crew is well and healthy.'    

Last radio contact with the Arctic Sea was on July 28 from the Dover Strait between Britain and France. Shortly afterwards an electronic signal showing its location was turned off.

Russia's domestic intelligence service, the FSB, was helping to investigate the mystery and its agents were at the offices of Solchart Arkhangelsk, which is listed as the ship's owner, Russian media reported.

Mikhail Boytenko, editor of Russia's respected Sovfracht maritime journal, said that the ship may have been carrying a secret cargo unknown to the vessel's owners or operators.

'I think there was probably some sort of secret cargo on this vessel, not criminal but secret,' he told Reuters.

'I don't think that it was pirates who took this vessel but it really smells of some sort of state involvement. This is real cloak and dagger stuff, like a (John) le Carre novel.'   

Relatives of the crew appealed to Prime Minister Vladimir Putin in an open letter and demanded a criminal investigation into the vessel's disappearance, Russian media reported, but have now stopped speaking publicly about the case.

Russian maritime journal Sovfracht reported on Thursday that a ship similar to the Arctic Sea had docked in the Spanish port of San Sebastian but the city, a popular seaside resort, only has a marina for small pleasure craft.

The port authority for the closest commercial port, Pasajes, said no such ship has docked. - Reuters