CHINA, though not specifically mentioned in the new US-Japan defence cooperation guidelines, will likely be wary of how they seem designed to target the emerging superpower, analysts say.
But they add that China may not react too strongly, given that it is keeping a close watch on its slowing economy, which it is seeking to boost through better ties with Japan.
Under the new guidelines, Japan would be allowed to aid allies, such as the United States, that come under attack, when previous arrangements largely restricted the deployment of the Japanese military to self-defence.
This strengthens US-Japan defence ties amid an American rebalancing towards Asia and territorial disputes in the region.
China would see itself as the obvious target of the new guidelines, Dr Hu Lingyuan, director of Fudan University's Centre for Japanese Studies, told The Straits Times.
"China will be wary because this affirms Japan's shift away from its pacifist post-war defence policy," Dr Hu said.
Japan's Prime Minister Shinzo Abe first announced proposed revisions to the policy last year.
China's Foreign Ministry spokesman Hong Lei yesterday gave a measured response to the updated US-Japan pact, which was initiated in 1978 to counter the threat from the Soviet Union.
Since the Cold War is over, how this enhanced cooperation develops is "worth watching", Mr Hong told reporters at a regular press briefing.
"The cooperation should ensure that it doesn't hurt third parties, including China, and ensure the region's peace and stability," he said.
But commentaries in Chinese state media warned about the instability the US-Japan move could trigger.
They charged that Tokyo has been reduced to being a pawn in big power relations between Beijing and Washington.
"A zero-sum game is the most damaging," said a commentary in the overseas edition of the People's Daily.
Despite China's unhappiness, it might, however, temper its reaction because it has one eye on its slowing economy, said Dr Zhou Yongsheng from China Foreign Affairs University.
Last week, Chinese President Xi Jinping held an amicable meeting with Mr Abe on the sidelines of a summit in Indonesia.
"Economically, China and Japan are hurting right now," Dr Zhou told The Straits Times.
"On this aspect, both sides are seeking a win-win solution."