Japan and South Korea have deployed their fighter jets after spotting Chinese military planes flying back and forth between the East China Sea and the Sea of Japan, according to Japanese and South Korean officials.
Japanese defence ministry officials said on Tuesday that eight Chinese jets flew over the Tsushima Strait in western Japan on Monday.
The officials said fighters from Japan's Air Self-Defense Force were scrambled.
The Chinese aircraft did not violate Japan's airspace, Japan's NHK World reported.
The ministry officials said the Chinese aircraft were six H-6 bombers, plus an early-warning plane and an intelligence-gathering plane.
They also said the Chinese planes headed northeast over the Sea of Japan and later made a U-turn and flew back to the East China Sea.
South Korea's Yonhap News Agency earlier reported that around 10 Chinese planes also entered the Korean air defence identification zone for five hours on Monday, prompting South Korea to launch "a sortie of fighter jets", and send a warning signal to the Chinese planes.
Yonhap said these planes flew near a submerged rock located off the southern coast of Jeju island "several times".
The submerged island, referred to by South Korea as Ieodo, is also included by China in its own air defence identification zone.
South Korea responded by sending 10 F-15 and F-16 fighter jets to the area.
Following the incident, The Japanese defence ministry said it was analysing the flights in order to determine their purpose.
In January of last year, two Chinese military planes were spotted over the Tsushima Strait, and, in August, three planes were seen going back and forth in the area.
China has been expanding its maritime activity.
Last month, its aircraft carrier made its first appearance in the Pacific Ocean.