On Monday, four Chinese patrol ships entered the water area bordering the Japanese territorial waters near the Senkaku Islands.

Then two of them entered the territorial waters of Japan and remained there until Tuesday morning, joined by the other two vessels shortly after.

A cannon was seen aboard one of the ships.

This is the first appearance of a Chinese ship with this kind of weapon on board in the Japanese territorial waters since the adoption of a law by Beijing in January allowing its coast guard to fire at foreign ships in waters it claims.

"It is extremely regrettable that the Chinese patrol ships have entered the territorial waters of our country, as well as their attempts to approach Japanese fishing vessels. This is completely unacceptable. We lodge a strong protest with the Chinese side through diplomatic channels," Kato said at a press conference.

In 2020, Japan registered 24 cases of Chinese ships' violation of its sea border and 333 cases of Chinese ships’ entrance into Japan's contiguous zone.

In mid-October, China's vessels entered Japan's territorial waters and left them only 57 hours later, marking a new record of the length of stay in the area.

The previous record was hit in July, when Chinese ships drifted in Japan's territorial sea for 39 hours.

The islands in question, known in China as the Diaoyudao islands, have long been an object of territorial disputes between China and Japan. Tokyo maintains it has had sovereignty over them since 1895 and Beijing claims that the islands are marked as a Chinese territory on Japanese maps circa 1783 and 1785.

Following World War II, the islands were controlled by the United States and handed over to Japan in 1972. China believes Japan illegally seized them.