A salvage vessel left Yokosuka port on Saturday for the East China Sea, where it will attempt to raise an unidentified ship that sank after a shootout with Japan Coast Guard patrol boats in December.

The 657-ton Shintatsu Maru, owned by a private salvage firm, is carrying an unmanned submersible vessel. The JCG commissioned the private salvage company to carry out the work, with the entire operation expected to take at least a month to complete.

The vessel will join the 498-ton Shinyo Maru, a salvage ship loaded with a manned submersible, on Tuesday evening in the waters where the suspected North Korean spy ship sank, JCG officials said.

Crew members in the manned submersible will guide the unmanned vessel in clearing the area around the ship, the officials said.

The clearing work is expected to take six days. Divers will then fix cables around the wreckage to raise the ship. , which lies at a depth of 90 meters.

The ship sank off Shanghai in China's exclusive economic zone, with China agreeing Tuesday to allow Japan to raise it.

Tokyo has conducted a series of underwater probes of the ship, and the government has said that its findings bolstered its belief that the ship was from North Korea.

Japan claims the vessel was either a spy ship or engaged in drug-running activities. North Korea has denied having any connection with the vessel but denounced its sinking as an "act of piracy."

Fifteen people were believed to have been on board the ship when it went down Dec. 22. Four bodies were recovered and the other crew members are presumed dead. Three JCG members were wounded in the incident.