Japan's top uniformed officer of the Defense Ministry said Thursday that recent activities by China's navy, including fighter jets taking off and landing on an aircraft carrier near the southern prefecture of Okinawa, are "a security concern."

Gen. Koji Yamazaki, chief of the ministry's Joint Staff, aired a strong sense of caution against the activities "held in waters close to Japan" with the aim of "bolstering navigating capabilities of the aircraft carrier in the Pacific region" when speaking at a press conference in Tokyo.

The remarks came as Self-Defense Forces vessels and aircraft have kept a vigilant watch for the Chinese navy's flotilla, including the aircraft carrier Liaoning, missile destroyers and a supply ship. In early May, they sailed from the East China Sea to the Pacific Ocean through waters between Okinawa's main island and Miyako Island.

The Liaoning was deployed about 150 kilometers south of Ishigaki Island in the prefecture at one point, according to the ministry, adding that fighter jets took off and landed on the vessel for a ninth straight day on Wednesday.

The Liaoning remained in the area as of Thursday, according to Yamazaki, saying he "took seriously" that the Chinese navy operates "near Taiwan and the southwestern region" of Japan.

Earlier this week, Chief Cabinet Secretary Hirokazu Matsuno told a press briefing that Japan would "closely monitor" the moves by the Chinese military.

China has been exhibiting increasing assertiveness in the East China Sea, including in Japanese territorial waters around the uninhabited Tokyo-controlled Senkaku Islands, which Beijing claims and calls Diaoyu.