The hijackers of a cargo ship that disappeared off the coast of France threatened to blow it up if their ransom demands were not met, Russian news agencies said.

Russia has arrested eight people on suspicion of hijacking the Arctic Sea off the Swedish coast and sailing it to the Atlantic Ocean, ending weeks of silence about the fate of a ship which has intrigued European maritime authorities.

Limited information from Russian officials has failed to satisfy sceptics who voiced doubts about whether the piracy actually took place or was a convenient cover story to conceal a possible secret cargo of arms or nuclear material.

"The crew members have already confirmed that the captors demanded a ransom and threatened to blow up the vessel if their orders were not obeyed," Interfax quoted a Russian defence ministry spokesman as saying.

"The crew members also claim that the people who seized the Arctic Sea were armed and got rid of their weapons when the [Russian navy ship] Ladny ordered the dry cargo carrier's crew to stop the vessel," he said.

Climbing gear, flares and a high-speed inflatable boat supposedly used in the hijack were found aboard the Arctic Sea, the spokesman said.

The agencies did not say what ransom was demanded.

The Maltese-registered, Russian-crewed vessel and its $1.3 million cargo of timber disappeared from radar screens three weeks ago, prompting speculation ranging from an attack by an organised crime gang to a top-secret spy mission.

The Malta Maritime Authority said on Tuesday, without elaborating, that the Arctic Sea had "never really disappeared", a comment which increased speculation that security services might have been involved in the affair.

Russia has said the eight detainees were citizens of Estonia, Latvia and Russia who on July 24 boarded the ship, forced the crew to change route and turned off its navigation equipment.

After heading through the English Channel in late July, radio contact was lost and the 4,000-tonne ship did not deliver its cargo to the Algerian port of Bejaia on August 4.

The Russian navy found the missing ship on Monday in the Atlantic Ocean near Cape Verde.

The official version of events was questioned by Yulia Latynina, a leading Russian opposition journalist and commentator.

"The Arctic Sea was carrying something, not timber and not from Finland, that necessitated some major work on the ship," she wrote in the Moscow Times newspaper on Wednesday.

During two weeks of repair works in the Russian port of Kaliningrad just before the voyage, the ship's bulkhead was dismantled so something very large could be loaded, she wrote.

"To put it plainly: The Arctic Sea was carrying some sort of anti-aircraft or nuclear contraption intended for a nice, peaceful country like Syria, and they were caught with it," she said.

- Reuters