Tokyo criticized Seoul on Friday over a South Korean minister’s use of the term “sex slaves” at a U.N. committee meeting in Geneva to describe the women who were forced to work in Japanese military brothels before and during World War II.

“Japan is of the view that the expression ‘sex slaves’ contradicts the facts and should not be used,” the Foreign Ministry said in a statement.

Foreign Minister Taro Kono told reporters after a Cabinet meeting that the use of the term on Thursday was “unacceptable and extremely regrettable.” An unknown number of the women, euphemistically called “comfort women” in Japan, were from the Korean Peninsula, and the issue has strained ties between the two Asian states.

The ministry referred to a 2015 bilateral agreement about the comfort women issue that said the term “sex slave” should be avoided.

The South Korean government, under President Moon Jae-in, has since found faults in the way the agreement was negotiated by his predecessor’s administration, but Tokyo has repeatedly called on Seoul to stick to it.

“The government of Japan continues to strongly urge (South Korea) to steadily implement the agreement as a ‘final and irreversible’ agreement,” ministry spokesman Norio Maruyama said in the statement.

According to the ministry, South Korean Gender Equality and Family Minister Chung Hyun-back used the term in a meeting of the U.N. Committee on the Elimination of Discrimination against Women.

She had been asked about a periodic report South Korea submitted to the committee about its progress on eliminating discrimination against women.

After Chung’s remarks, Junichi Ihara, Japan’s ambassador to the international organizations in Geneva, lodged a protest in a telephone call to his South Korean counterpart Choi Kyong-lim, who said he would pass the message on to Seoul.

South Korean Foreign Minister Kang Kyung-wha is scheduled to speak at a U.N. Human Rights Council meeting in Geneva starting on Monday, and she may bring up the comfort women issue there.