Nationalist Prime Minister Shinzo Abe paid an inflammatory visit to the Yasukuni war shrine Thursday, angering China which accused Japan of whitewashing a history of warmongering and said it must "bear the consequences".
Seoul also blasted the "anachronistic" move and Tokyo's chief ally the United States declared itself "disappointed" with an act that will "exacerbate tensions with Japan's neighbours".
Abe described his visit, which came days after he gave Japan's military its second consecutive annual budget bump, as a pledge against war and said it was not aimed at hurting feelings in China or South Korea.
Yasukuni Shrine is the believed repository of around 2.5 million souls of Japan's war dead, most of them common soldiers, but also including several high-level officials executed for war crimes after World War II.
South Korea and China see it as a symbol of Tokyo's unrepentance for the horrors of last century and say it whitewashes the country's warmongering past.
"Some people criticise the visit to Yasukuni as paying homage to war criminals, but the purpose of my visit today... is... to renew the pledge that Japan must never wage a war again," Abe said in a statement released after the visit.
"It is not my intention at all to hurt the feelings of the Chinese and Korean people. It is my wish to respect each other's character, protect freedom and democracy, and build friendship with China and Korea with respect."
The visit came exactly 12 months after he took power, a period in which he has formally met neither China's President Xi Jinping, nor Korea's President Park Geun-Hye.
Ties with Beijing were bad before Abe took office, with the two countries crossing diplomatic swords over the ownership of uninhabited islands in the East China Sea.
The dispute has ratcheted up this year, with the involvement of military aircraft and ships, leaving some observers warning of the danger of armed conflict.
Beijing wasted no time in slamming Abe's move, which came the day Xi and other senior Chinese leaders visited the mausoleum of Mao Zedong, the hardline communist who led China through the disastrous Great Leap Forward, in which up to 45 million people died.
"The essence of Japanese leaders' visits to the Yasukuni shrine is to beautify Japan's history of militaristic aggression and colonial rule," foreign ministry spokesman Qin Gang said.
Chinese foreign ministry official Luo Zhaohui called the visit "absolutely unacceptable to the Chinese people" and cautioned Japan "must bear the consequences arising from this".
The last incumbent Japanese prime minister to visit the shrine was Junichiro Koizumi in 2006. His repeated pilgrimages badly soured relations with China, despite the important economic and trade ties that bind the two countries.
The foreign ministry in Tokyo said it wanted to stress Abe "visited Yasukuni Shrine in a purely personal capacity (and)... not... to pay homage to war criminals".
However, China and South Korea, both victims of Japan's 20th century aggression, say no such distinction exists.
"We can't help deploring and expressing anger at the prime minister's visit to the Yasukuni shrine... despite concerns and warnings by neighbouring countries," Seoul's culture minister Yoo Jin-Ryong told reporters.
"The visit... is an anachronistic behaviour that fundamentally damages not only relations between the South and Japan but also the stability and cooperation of northeast Asia," he said.
Washington, which has to tread a careful line between supporting its chief regional ally in the face of China's rise to global superpower and emboldening a prime minister many observers see as a hot-headed trouble-maker, offered qualified criticism.
"Japan is a valued ally and friend. Nevertheless, the United States is disappointed that Japan's leadership has taken an action that will exacerbate tensions with Japan's neighbours," a written statement said.
"We take note of the Prime Minister's expression of remorse for the past and his reaffirmation of Japan's commitment to peace."
Abe's forthright views on history -- he has previously questioned the definition of "invade" in relation to Japan's military adventurism last century -- have raised fears over the direction he wants to take officially-pacifist Japan.
He has spoken repeatedly of his desire to tweak the US-imposed constitution and is pushing to broaden the role of the military to permit "collective self-defence", allowing Japanese troops to come to the aid of allies.
Takehiko Yamamoto, a professor of international relations at Tokyo's Waseda University, said the visit was "an act of folly" that was certain to make a bad situation worse.
"It is perfectly possible his visit will fuel worries in Washington over a possible rise of militarism and a shift to the right in Japan," he said.