A crew member of a high-speed vessel believed to be North Korean used a rifle to threaten a Japan Coast Guard patrol boat last month in the Sea of Japan, coast guard officials revealed Friday.

The government has lodged a strong protest with North Korea through the Japanese Embassy in Beijing over the incident, Chief Cabinet Secretary Yoshihide Suga said at a regular news conference.

The incident took place near an area in the Sea of Japan known as Yamatotai, which lies within the country's exclusive economic zone. Yamatotai is fertile fishing ground for squid and other types of fish, according to the officials.