Japan's main opposition leader chided Prime Minister Shinzo Abe for conservative statements on history and voiced fear he could be a "destabilising" factor in East Asia.
During a visit to Washington, Banri Kaieda, president of the opposition Democratic Party of Japan, said he remained fully committed to the country's past expressions of regret for its wartime behaviour.
Kaieda said that the Abe government's remarks and actions had alienated Japan's neighbours as well as its US and European allies by "fuelling suspicions that Prime Minister Abe may be a historical revisionist".
Speaking at the Brookings Institution think tank on Tuesday, Kaieda said: "I clearly reject historical revisionism and will oppose it." He said his party would "safeguard the mature democracy fostered by post-war Japanese society".
He went on: "Domestically, the Abe administration has now made its authoritarian tendencies clear and internationally, the Abe administration could move beyond the realm of healthy nationalism and become a destabilising factor in East Asia."
In December Abe isited the Yasukuni Shrine, which honours 2.5 million Japanese war dead including convicted war criminals from the second world war. The move outraged China and South Korea and led to a rare public rebuke by the United States, Japan's key ally.
Abe, whose grandfather was arrested but not prosecuted as a war criminal, is known for his conservative views. While in opposition, he questioned whether imperial Japan forced women into sexual slavery, although he has since indicated he will not revoke a landmark 1993 apology to the so-called "comfort women".
Kaieda voiced hope for strong relations with both Asian neighbours' and the US, saying that Japan could still voice concerns about China while expressing remorse for the past.
Kaieda portrayed himself as an ideological soulmate of President Barack Obama, saying that his party shared US Democrats' principles of social inclusion.
The Democratic Party of Japan "wants an open nation that's cosmopolitan, multicultural; an open nation with no gender discrimination. We will thoroughly oppose xenophobia," he said.