Top Russian officials admitted yesterday that a cargo ship hijacked under murky circumstances in the Baltic Sea may have been carrying a suspicious cargo, after initially playing down such reports.

Speculation has been raging that the Arctic Sea - a ship that was seized by pirates near Sweden last month and vanished for weeks before being recaptured by the Russian navy - may have held weapons or even nuclear materials.

The Maltese-flagged ship with a crew of 15 Russian sailors was officially heading to Algeria with a cargo of timber.

But Moscow's top investigator, Alexander Bastrykin, cast doubt on that in a newspaper interview.

"We do not rule out the possibility that the Arctic Sea transported something other than wood," Mr Bastrykin told the official government daily Rossiyskaya Gazeta.

"This is why we detained the crew, as we must figure out if any one of them was involved in those events," added Mr Bastrykin, who heads the investigative committee of Russian prosecutors.

The detention of 11 Arctic Sea sailors by Russian authorities and media reports that they have not been allowed to communicate with their families have fuelled speculation of a cover-up.

Mr Bastrykin pledged that "in a week and a half, we will give complete information" about the incident.

Separately, the head of the Russian military said that Moscow did not know if the ship was carrying any cargo other than wood.

"We do not know what it is carrying, we only know there is wood and whatever else it is carrying must be clarified by the investigation," Nikolai Makarov, chief of Russia's general staff, told reporters during a visit to Mongolia.