Japanese and South Korean military aircraft have flown through disputed air space over the East China Sea in defiance of a demand from Beijing to identify themselves to Chinese authorities.

The move was seen as a challenge to a new Chinese air defence zone that has increased regional tensions and sparked concerns of an unintended clash.

The US has also defied China's demand by flying two unarmed B-52 bombers over the islands on Tuesday without informing Beijing.

Tensions have increased 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 US have sharply criticised 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 US does not take a position on the sovereignty of the islands but recognises 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.

China has 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," Mr 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.