Prime Minister Shinzo Abe denounced a top South Korean lawmaker’s comments about Emperor Akihito as “extremely inappropriate,” ratcheting up already high tensions between the two neighbors.

Abe told the Diet on Tuesday that Japan asked South Korea to apologize for National Assembly Speaker Moon Hee-sang’s remarks last week describing Emperor Akihito as “the son of the main culprit of war crimes.” Moon made the statement in a Bloomberg interview Thursday in which he urged an imperial apology to resolve a dispute over the “comfort women.”

The term comfort women is a euphemism used to refer to women who provided sex, including those who did so against their will, for Japanese troops before and during World War II.

“When I read these remarks, I was really surprised,” Abe told lawmakers during the Lower House Budget Committee meeting in Tokyo. “Our country immediately conveyed to South Korea via the diplomatic route that Speaker Moon’s comments were extremely inappropriate and most regrettable. We protested strongly and called for an apology and a retraction.”

Top government spokesman Yoshihide Suga also told a news conference the same day that the remarks “were extremely inappropriate and deplorable,” adding Tokyo has lodged a “stern protest” with Seoul and demanded an apology through diplomatic channels.

The South Korean government has told Japan that the report did not reflect Moon’s “true intentions” to seek an improvement in bilateral ties, according to Suga.

The South Korean Ministry of Foreign Affairs said Monday that Moon’s remarks were intended to emphasize the suffering of the victims and that the country was committed to “future-oriented” ties. “Japan needs to show sincerity for honor, dignity and to heal the emotional pain of the victims based on a victim-centered approach,” the ministry said.

The ministry confirmed Tuesday that it had received Japanese requests for an apology Sunday and Monday, but didn’t immediately respond to Abe’s latest remarks.

The row comes at a time when bilateral ties have increasingly chilled over wartime issues related to Japan’s colonial rule of the Korean Peninsula between 1910 and 1945, as well as disputes over military activities.

A standoff over the comfort women issue has been intensifying after South Korea said in November last year that it will dissolve a Japanese-funded foundation set up under a 2015 agreement designed to finally and irreversibly settle the issue.

The South Korean assembly speaker’s interview came before the 85-year-old Emperor abdicates on April 30, the first living Japanese monarch to do so in about 200 years.

Bilateral ties have also soured over South Korean top court rulings ordering Japanese companies to pay compensation for wartime labor, and a dispute over a South Korean navy vessel’s alleged locking of a fire-control radar on a Japanese patrol plane in December.

Emperor Akihito expressed his “deepest regret” on May 24, 1990, over the “sufferings” Koreans experienced during Japan’s colonial rule.

Addressing an Imperial Palace banquet for the visiting South Korean president, Roh Tae-woo, the Emperor said: “I think of the sufferings your people underwent during this unfortunate period, which was brought about by my country, and cannot but feel the deepest regret.”

The Emperor’s statement was apparently aimed at putting an end to the long-standing dispute between the two countries over South Korea’s demand for a “clear-cut apology” by the emperor for the colonial rule.

On Sunday, Foreign Minister Taro Kono had cautioned Moon against making divisive remarks, without publicly demanding an apology.

“We don’t yet know how South Korea will deal with this, but we expect a sincere response,” Kono told the Diet on Tuesday.

Moon was in Washington, where he met with Deputy U.S. Secretary of State John Sullivan on Monday. They pledged to strengthen trilateral cooperation with Japan, the State Department said.

“It only takes one word from the prime minister, who represents Japan — I wish the emperor would do it since he will step down soon,” Moon said in the Bloomberg interview. “Isn’t he the son of the main culprit of war crimes?”