China has expressed concern over a US-Japanese plan to deal with a possible military conflict across the Taiwan Straits, which also indirectly aims to curb Beijing's growing might.

"We express grave concern," Chinese Foreign Ministry spokesman Liu Jianchao said.

Taiwan is an inalienable part of Chinese territory, and whatever considerations between Japan and the United States should firmly adhere to the 'one-China' policy, he said.

"China-Japan and China-US relations are improving, and we hope relevant parties can do more positive contribution to the development of bilateral ties," Liu said.

The secessionist forces in Taiwan are 'the greatest threat to peace and stability across the Taiwan Straits', he said.

China and Taiwan split in 1949 at the end of a civil war but Beijing still claims the self-ruled island as part its territory.

China has repeatedly threatened to invade if Taiwan declares independence or drags its feet on reunification talks.

Moreover, the Chinese parliament has also passed an anti-secession law in 2005 to foil Taiwan's bid to seek independence.

Washington switched diplomatic recognition from Taipei to Beijing in 1979.

However, the United States continues to be the main ally and main defence supplier of the cash-rich island of 23 million people.

Washington and Tokyo will discuss a contingency plan in case of a 'crisis' situation arising in areas around Japan, including the Taiwan Straits, media reports from Tokyo said.

The report quoted several sources familiar with Japan-US military cooperation as saying the two sides have reached consensus on the necessity for such a contingency plan and will soon begin discussing the details.

Meanwhile, Chinese experts on international studies warn that the strengthening of the US-Japanese alliance is targeted at China and would further complicate the nation's security environment.

A senior researcher with the Chinese Academy of Social Sciences, Tao Wenzhao, noted that the alliance aims to act as 'a policeman' in the Taiwan Straits and play a dominant role in the region.

Wu Xinbo, a professor with the Centre of America Studies at Fudan University, pointed out that enhanced security ties between Washington and Tokyo since the mid-1990s have reshaped the East Asian security environment; and Japan has showed increased assertiveness and willingness to work militarily with the United States.

He said interaction between Washington and Tokyo on the Taiwan issue has been increasing, with Tokyo more actively consulting and coordinating with Washington in its 'relations' with the island province, from listing Taiwan as a common strategic objective to working on a joint war plan for the Taiwan Straits.

Japanese Prime Minister Shinzo Abe, without referring to the 'Taiwan contingency plan', said on Thursday that his country would pursue assertive foreign policies and strengthen its ties with the United States and Europe in response to new security threats in the region.

On Tuesday, China also demanded the United States to take practical action to contain 'Taiwan independence', which is posing the biggest threat to peace and stability in the Asia-Pacific region.