Tokyo/Seoul: Japanese and South Korean military aircraft flew through disputed air space over the East China Sea without informing China, Reuters reported quoting officials on Thursday.
The move is considered that the two nations do not approve the new Chinese air defence zone that has increased regional tensions and sparked concerns of an unintended clash.
The move came after Tokyo's close ally Washington defied China's demand that airplanes flying through its unilaterally announced zone identify themselves to Chinese authorities, flying two unarmed B-52 bombers over the islands on Tuesday without informing Beijing.
Tensions have ratcheted up since Beijing's weekend announcement of the zone that includes the skies over islands at the heart of a feud between Japan and China, and its demand that planes flying in the area first notify Chinese authorities.
Japan and the United States have sharply criticized the move, which some experts said was aimed not only at chipping away at Tokyo's control of the islands, known as the Senkaku in Japan and the Diaoyu in China, but also at challenging US dominance in the region.
The United States does not take a position on the sovereignty of the islands but recognizes Tokyo's administrative control and has assured Japan that the US-Japan security pact covers them.
The developments are expected to dominate US vice president Joe Biden's visit to Japan, China and South Korea next week.
Also on Thursday, China rejected South Korea's demand for the repeal of the zone, but appeared to soften its demand that commercial aircraft tell its military authorities of any plans to transit the area. Japan's two biggest airlines have already begun defying that order.
"The East China Sea Air Defence Identification zone is not aimed at normal international flights. We hope that relevant countries' airlines can proactively cooperate, so there is more order and safety for flights," China's Foreign Ministry spokesman Qin Gang told reporters.
Japan's chief cabinet secretary Yoshihide Suga said naval ships and patrol planes have been operating in the East China Sea and would continue to do so.
"They are carrying out surveillance activity as before in the East China Sea, including the zone," Suga told a news conference, adding there has been no particular response from China. "We are not going to change this (activity) out of consideration to China," he added.