Japan's military also said it had scrambled jets in response to flights over the Sea of Japan, also known as the East Sea, by Russian and Chinese aircraft.

Seoul's Joint Chiefs of Staff (JCS) said the Chinese H-6 bombers repeatedly entered and exited the Korea Air Defence Identification Zone (KADIZ) near South Korea's southern and northeastern coasts early Wednesday.

Hours later they returned to the zone from the East Sea, accompanied by Russian warplanes including two Su-35 fighter jets and four TU-95 bombers, it added.

All the warplanes eventually left the zone and none violated South Korea's airspace, Seoul said.

An ADIZ is an area wider than a country's airspace in which it tries to control aircraft for security reasons, but the concept is not defined in any international treaty.

"Our military deployed air force fighter jets even before Chinese and Russian aircraft entered the KADIZ to take tactical measures in case of contingency," the JCS said in a statement.

Beijing and Moscow appeared to have "engaged in a combined air exercise", Seoul's Yonhap news agency reported, citing unnamed "observers".

Japan's Joint Staff said two Chinese H-6 bombers "entered the Sea of Japan and then flew north" on Wednesday morning.

"Around the same time, what appears to be two Russian aircraft flew south over the Sea of Japan and then turned around," it said, adding that it had scrambled jets in response.

The incident comes as Washington pushes China, North Korea's most important ally, to use its influence to help rein in Pyongyang, which has conducted a record-breaking blitz of missile launches this year.

Chinese President Xi Jinping recently told Kim Jong Un that he was willing to work with the North Korean leader for "world peace".

Pyongyang earlier this month fired an intercontinental ballistic missile in one of its most powerful tests yet, declaring it would meet perceived US nuclear threats with nukes of its own.

The United States has accused Beijing and Moscow of protecting Pyongyang from further punishment.

The two countries in May vetoed a US-led effort to tighten sanctions on North Korea in response to the North's earlier missile launches.