NHK has learned a North Korean vessel equipped with what appears to be a cannon was first detected in waters near Japanese fishing boundaries in the Sea of Japan.
Yamatotai is located off the coast of Noto Peninsula, central Japan, within Japan's exclusive economic zone. It is one of Japan's key fishing areas.
Illegal operations by North Korean and Chinese fishing boats have been seen in the area in recent years. North Korea's fishing and government ships have also been seen navigating in the waters.
Sources told NHK that the Japan Coast Guard first detected last year a ship belonging to North Korean authorities, equipped with what looked like a cannon and anti-aircraft radar, near Yamatotai for the first time. That ship was believed to be of a patrol type, and has often been seen in the area since.
Sources said that in light of the vessel's shape, equipment and traces of letters painted on its hull, what appeared to be a North Korean patrol vessel was likely a former Chinese government ship.
Most North Korean government ships that had been seen navigating there before were of a fishing or cargo type.
The Japan Coast Guard has enhanced vigilance in the waters ahead of the peak fishing season.
Ichijo Masahiro, who has held a senior position at the Japan Coast Guard, says North Korea seems to be serious about going on patrol against Japanese and Chinese ships as the country claims the area.
He called on Japanese fishers to operate safely taking into consideration the current situation around the waters.
He added that they do not need to be too anxious as the Japan Coast Guard is ready to respond to various situations.