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.

The coast guard has been strengthening its crackdown on illegal fishing by North Korean ships in the area.

“We will respond resolutely in cooperation with the Japan Coast Guard to (foreign ships illegally) entering Japan’s EEZ,” fisheries minister Taku Eto said at a news conference.

According to the officials, the Japan Coast Guard received a request to dispatch a boat on the morning of Aug. 23 from a ship of the Fisheries Agency patrolling in the area, which said a high-speed vessel of unknown nationality was approaching.

The coast guard found a high-speed vessel and what appeared to be a cargo ship with the North Korean national flag some 378 kilometers off the Noto Peninsula in Ishikawa Prefecture, the officials said. The two vessels subsequently left the area.

But at around 9 a.m. on Aug. 24, a coast guard patrol boat found two ships that looked identical to the ones spotted a day earlier.

One of them was a green high-speed vessel hoisting what appeared to be a North Korean Navy flag, which came as close as 30 meters to the coast guard boat at one point, the officials said.

Three crew members in camouflage clothes were aboard the high-speed vessel with one pointing a rifle at the coast guard boat and another filming the encounter. No shots were fired and the vessel left the area, the officials said.