A salvage firm plans to use an unmanned robot to again try to raise a sunken tour boat from the seabed off Shiretoko Peninsula in the northern Japanese prefecture of Hokkaido.

On Monday, salvage workers lifted the "KAZU I" sightseeing boat up to around 20 meters below the surface, after deep-sea divers attached five belts to the vessel.

But, the boat fell back down to the seabed on Tuesday, as it was being towed by a barge toward the town of Shari.

The vessel was later confirmed to be lying on the seabed at a depth of about 180 meters, 11 kilometers to the west of Utoro Port in Shari. No major damage has been detected.

Coast guard officials say two belts attached to the rear side were cut. They say they do not have any other details about what caused the vessel to drop down.

The salvage firm now plans to use an unmanned undersea robot to put belts on the boat on Wednesday. The firm then intends to pull the vessel up toward the surface and attach it to the barge the following day.

It will then tow the boat to an area one kilometer off Utoro, and lift it onto the barge. The company plans to unload the vessel at Abashiri Port as early as Friday.

The salvage firm does not intend to have deep-sea divers carry out the work this time.

The "KAZU I" sank during a sightseeing tour on April 23. Of the 26 people on board, 14 have been confirmed dead. The other 12 remain missing.