Japan has begun work to salvage a suspected North Korean spy ship which sank after a gun battle with the Japanese coastguard last year.

The operation comes amid reports that Japanese divers have spotted an anti-aircraft missile and other weapons on the boat.

A spokesman for the Japan coastguard said the operation, which has been delayed for two days due to bad weather, will begin with a check for obstacles to the salvage.

The vessel is lying on the seabed some 90 metres (297 feet) underwater about 390 kilometres (240 miles) west-northwest of Amami-Oshima island in south-western Japan.

Survey

The coastguard spokesman explained that an unmanned submersible will survey the wreckage to check for obstacles to the salvaging work, while a manned mini submarine will check the seabed surrounding the wreckage, and will monitor the unmanned machine.

Japan wants to find out where the boat came from

The survey is scheduled to take about six days, and the entire salvage operation is scheduled to take until at least the end of July.

Unconfirmed reports in the Japanese media on Wednesday quoted government sources as saying that a Russian surface-to-air heat-seeking missile, portable anti-tank grenade launchers and machine guns have already been seen on the ship.

A Japanese official refused to confirm the reports, suggesting they were "speculation".

North Korean links

North Korea's Foreign Ministry has denied having anything to do with the boat, although there are reported links with the secretive state.

Japanese officials have said that pictures showed the boat's crew using shoulder-held rocket launchers, similar to Soviet-made weapons known to be in North Korea's arsenal.

And four bodies of the 15 crew on board have been recovered wearing items with labels in Korean.

The mystery boat sank on 22 December after a six-hour chase and fire fight with Japanese patrol ships.

The vessel, having failed to heed Japanese warning shots and an order to stop, fled in the direction of China before it sank.

Japan has raised the possibility that the crew scuttled the ship.

Delicate diplomacy

The ship sank near Chinese territorial waters, in what Beijing calls its exclusive economic zone, but where Japan claims the right to protect its fishing and mineral resources.

China gave Japan permission last week to raise the ship, provided both countries discussed compensation for what China says is damage caused to its fisheries by Japanese patrols.

The Japanese coastguard says the inquiry is a "criminal investigation" into attempted murder, as the vessel had returned fire after the coast guard fired warning shots.