China completed a second phase of joint air patrols with Russia over the Western Pacific on Wednesday, following flights on the previous day over the Sea of Japan and East China Sea, sparking concerns in Japan over national security.

Both South Korea and Japan had scrambled fighter jets on Tuesday, when the Chinese and Russian joint patrols got underway as part of their militaries annual cooperation plan.

The flights are "serious concern" for Japan's national security, and those concerns had been conveyed to China and Russia through diplomatic channels, Chief Cabinet Secretary Hirokazu Matsuno told a news conference in Tokyo on Wednesday.

"Such repeated joint flights by the strategic bombers of the two countries in the vicinity of our country signify the expansion of activities in the vicinity of our country, and are clearly intended to act arbitrarily at our country," Matsuno, the government's top spokesperson, said.

On Tuesday, South Korea scrambled fighter jets after four Russian and four Chinese military aircraft entered its air defence zone in the south and east of the Korean peninsula.

Japan's military said it had scrambled fighter jets after verifying that two Russian bombers had joined two Chinese bombers over the Sea of Japan and flown together as far as the East China Sea on Tuesday.

The latest round of joint air patrols were sixth conducted by Russia and China since 2019,and they come at a time of rising tensions in Asia Pacific, as both China and the United States have ramped up military activity in the region.