Japan has protested to North Korea after a Japanese patrol vessel spotted an armed boat believed to be from there allegedly fishing illegally, an official said Thursday.

The crew pointed a gun at the Japanese fisheries vessel, forcing it to withdraw, Chief Cabinet Secretary Yoshihide Suga told a news conference. The incident happened last Friday about 500 kilometers (310 miles) west of Japan's northern coast in an area Tokyo claims as its exclusive economic zone, Suga said.

Suga said that Japan determined the boat was North Korean by observing and analyzing its crew members as well as other details.

The Japanese vessel quickly left the area because of safety concerns, he said.

Suga refused to discuss the North Korean boat's activity or intentions except to say that it was operating illegally. Kyodo News service said the boat was allegedly fishing squid illegally and that it chased the Japanese vessel for more than 10 minutes.

The area is known as rich fishing grounds and Japanese authorities have been patrolling it for illegal operations.

Japan's protest was lodged via North Korea's embassy in Beijing.