An unidentified ship that was spotted in Japan's exclusive economic zone last month was later found to belong to the North Korean government, Chief Cabinet Secretary Katsunobu Kato said Wednesday.
The incident prompted Tokyo to lodge a protest with Pyongyang, the top government spokesman said.
According to Kato, a Fisheries Agency ship that was patrolling the Sea of Japan to the west of an area known as Yamatotai spotted the unidentified ship on Sept. 29 at around 8 a.m.
Upon further analysis, the ship was deemed to belong to the North Korean government, he said.
Yamatotai offers fertile fishing grounds for squid and other types of marine life, and has often become a target for illegal fishing by ships believed to be coming from North Korea and China.
It is not unheard of for North Korean government ships to make appearances in the area, sometimes threatening Japanese ships sent to intercept them.
In August last year, a high-speed vessel armed with a gun and flying what appeared to be a North Korean Navy flag threatened to fire at a Japan Coast Guard ship before leaving.