US Vice President Joe Biden has urged Japan and China to lower tensions that have spiked since Beijing announced an air defence zone over disputed islands in the East China Sea, while repeating that Washington was worried by the move.

The United States has made clear it would stand by treaty obligations that would require it to defend the Japanese-controlled islands, but is also reluctant to get dragged into any military clash between the Asian rivals.

Biden will meet Japanese Prime Minister Shinzo Abe on Tuesday before flying to China the next day as part of an Asian trip in which he will seek a delicate balance between calming tensions over the zone while backing key ally Japan.

"We remain deeply concerned by the announcement of a new Air Defence Identification Zone," Biden said in written answers to the Asahi daily newspaper.

"This latest incident underscores the need for agreement between China and Japan to establish crisis management and confidence building measures to lower tensions."

Zone rejected

Japan reiterated on Tuesday that Tokyo and Washington had both rejected Beijing's move to set up the zone - despite the fact that three US airlines, acting on government advice, are notifying China of plans to transit the area.

"We and the US have the same stance of not recognising this ADIZ," Chief Cabinet Secretary Yoshihide Suga told a news conference.

"We firmly confirm this."

Washington said over the weekend that the advice to US airlines did not mean U.S. acceptance of the zone, and last week sent two B-52 bombers into the area without informing China.

The Japanese and South Korean governments have advised their airlines not to submit flight plans in advance, which China has demanded from all aircraft since it announced the creation of the zone on November 23.

Source: Agencies