In a highly unusual remark for a Japanese official, Nobushige Takamizawa, director general of the defence ministry's defence policy bureau, said a contingency over Taiwan would be "a security matter for Japan."
"Because it would be a seriously significant matter for our country, the Self-Defence Forces would obviously step up their alert and surveillance activities before judging whether the contingency is happening in our so-called surrounding area," he told a gathering of ruling party lawmakers.
"It is not an issue of the Japan-US defence alliance. It is a security matter for Japan," he said, adding that Japan would have to "take appropriate actions" in a cross-strait confrontation.
In 2005, the United States and Japan declared Taiwan to be a common security issue for the Pacific allies, angering China which considers Taiwan to be part of its territory.
Japan has been working since 2006 to repair relations with China which have been tense over war memories.
Chief Cabinet Secretary Nobutaka Machimura, the top government spokesman, declined to directly comment on Takamizawa's remarks but said that China-Taiwan relations were unchanged.
"Our position remains the same that the both parties directly involved should peacefully resolve the situation through dialogues," Machimura said.
The post-World War II Japanese constitution denounces war and bans Japan from ever using force to resolve international conflicts, leading the country to call its military the Self-Defence Forces.
However, domestic law allows the Japanese military to offer logistical help, such as providing fuel and supplies, to US forces in case of contingencies in vaguely defined "surrounding areas."
Machimura said that "surrounding areas" was not a geographical concept but referred to situations that could directly affect Japan's security.
The United States is committed to defending both Taiwan and Japan, where it stations more than 40,000 troops. China considers Taiwan a territory awaiting reunification, by force if necessary.