Three Japanese sailors have suffered minor injuries after their submarine was damaged in a collision with a merchant vessel.
Japan's defence ministry said the incident involving the 84-metre Japanese Maritime Self-Defense Force sub Souryu occurred on Monday off Cape Ashizuri, Shikoku.
The Souryu scraped the hull of a cargoship as it surfaced, damaging its antenna mast and other upper parts.
A photo released by the Japan Coast Guard shows a tower fin buckled.
Government spokesman Katsunobu Kato said at a press conference an unnamed ship believed to be the one involved in the collision had been contacted by the coastguard.
The vessel reported that no impact was felt and it does not appear to have sustained any damage, however.
Crew unaware of any incident
Unnamed sources told Kyodo News the vessel involved may have been the 93,000-dwt bulker Ocean Artemis (built 2011).
The vessel's technical manager Huayang Maritime Center of Beijing referred TradeWinds to another company office, which could not immediately be contacted.
The submarine was involved in routine training and its sailors saw the cargo ship through its periscope, but was unable to avoid it, according to defence minister Nobuo Kishi.
The navy ship's communications equipment was damaged in the accident so reporting of the incident was delayed. The crew had to use a mobile phone to contact authorities.
Japanese navy commander Hiroshi Yamamura apologised to the Japanese people and the owner of Ocean Artemis for the incident.
The sub was able to carry on sailing and arrived at Kochi port late on Monday.
The Japan Coast Guard said it would open an investigation. The Japan Transport Safety Board has already begun its own probe.
The Soryu is a 2,950-gt diesel-electric submarine with a usual crew of about 65.
Adam Corbett contributed to this article.