Chinese military fighter jets twice locked their fire-control radar on Air Self-Defense Force aircraft over international waters southeast of Okinawa Prefecture over the weekend, the Defense Ministry in Tokyo said Sunday, an account Beijing denied as ties between the neighbors continue to fray.

Defense Minister Shinjiro Koizumi told a news conference early Sunday that the “dangerous acts” were “extremely regrettable,” adding that Japan had “lodged a strong protest” with the Chinese side and demanded strict measures to prevent a recurrence.

Chinese J-15 fighter jets launched from the Liaoning aircraft carrier “intermittently directed their radar” at ASDF F-15 fighters on Saturday, the ministry said. The F-15s had been scrambled as a response to a potential airspace intrusion, and came as the Chinese carrier and three missile destroyers were holding exercises.