"Japan's goal to significantly increase defense investments will... strengthen and modernize the US-Japan Alliance," Sullivan said on Friday.

In its largest defense shake-up in decades, Japan vowed to raise security spending to two percent of GDP by 2027, reshape its military command, and acquire new missiles that can strike far-flung enemy launch sites.

The move includes the purchase of as many as 500 US-made Tomahawk missiles.

"Fundamentally strengthening our defense capabilities is the most urgent challenge in this severe security environment," Prime Minister Fumio Kishida said last week.

US Secretary of Defense Lloyd Austin said the move showed "Japan's staunch commitment to upholding the international rules-based order and a free and open Indo-Pacific."

Japan warned China poses the "greatest strategic challenge ever" to its security, with a record volley of North Korean missile launches in recent months, including over Japanese territory, also causing alarm.

While Japan once called for enhanced ties and cooperation with Russia, it now warns that Moscow's military posturing in Asia and cooperation with China are "a strong security concern."

Japan has joined Western allies in imposing sanctions on Moscow over its invasion of Ukraine, sending relations into a deep freeze.