Japan's defence and foreign ministers are in Washington for key security talks with US counterparts.
There is speculation that one topic on the agenda is the Taiwan Strait, an important flashpoint in the region.
According to a draft proposal seen by US and Japanese press, Taiwan will be identified as a mutual security concern - a move set to anger its rival, China.
But a Japanese Foreign Ministry press spokesman refused to confirm this ahead of Saturday's talks.
Japanese Foreign Minister Nobutaka Machimura and Defence Minister Yoshinori Ono will join with US Secretary of State Condoleezza Rice and Defence Secretary Donald Rumsfeld in declaring security in the Taiwan Strait as a "common strategic objective", according to the Washington Post.
The revision, which may be announced as part of a joint statement after the ministers' meeting, reflects growing concern in Washington and Tokyo about China's increasing military power.
A Japanese Foreign Ministry spokesman would only confirm that Japan had long sought "a common strategic understanding of the [Asia-Pacific] region".
He denied that such a move would anger China, which views Taiwan as part of its territory and has threatened to invade Taiwan if it ever declared formal independence.
"We are not trying to call China a threat or anything. It's just that we need to review the whole situation," he said.
Flashpoint
The issue of Taiwan is sensitive because, if conflict did flare between China and Taiwan, the US has made clear that it might come to Taiwan's defence.
Japan's role in any conflict has never been spelled out, although thousands of US troops are stationed at military bases in Japan.
Its pacifist constitution currently limits the level of military assistance it could give the US.
But the statement on Saturday could pave the way for Japan to extend co-operation as much as it legally can, the Washington Post quoted Japanese officials as saying.
The talks in Washington come at a time of strained diplomatic relations between Japan and China.
Japan was shaken by the intrusion of a Chinese submarine into its waters in November, and in new Japanese defence guidelines released in December, China's military build-up was named as a key area of concern for the first time.
Also on the agenda of Saturday's talks will be North Korea, whom both the US and Japan are keen should return to talks on its nuclear programme, and the realignment of US forces in Japan.
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