A sonar search that began Monday for an unidentified ship that sank in the East China Sea in December after a firefight with Japanese patrol vessels has picked up telltale echoes from the seabed where the vessel is believed to have gone down, Japan Coast Guard officials said.

The ship, suspected of being a North Korean spy vessel, is believed to be sitting some 390 km west of Amami-Oshima Island in Kagoshima Prefecture in waters some 90 meters deep, the officials said.

Coast guard officials in charge of the sonar sweep reported in the afternoon that the 550-ton survey ship Kaiyo, towing an ultrasonic probe, picked up signals from the seabed that resonated differently from the sea floor, the Coast Guard said.

The Izu, a patrol vessel, will deploy a remote-controlled underwater TV camera today to check the vicinity of the echoes. If the ship is found, its condition will be surveyed and its identity checked, if possible, the officials said.

Weather-permitting, the search is scheduled to continue until Friday, they said.

The ship sank in China's exclusive economic zone on Dec. 22 during a chase from Japanese waters.

Around 15 people are believed to have been on board at the time. The Japan Coast Guard shortly after the shootout retrieved two bodies, and the rest are believed to have gone down with the ship.

Tokyo believes the ship was North Korean and was engaged in espionage or drug-running activities.

North Korea denies any links with the vessel but has also denounced its sinking as an "act of piracy."

Chief Cabinet Secretary Yasuo Fukuda said Friday that Japan had informed China of its investigation plans.

Chinese authorities were reportedly reluctant to let Japan search for and salvage the vessel in its economic zone, but recently gave Japan tacit approval of its plan.