Two members of the United States House of Representatives said Thursday they have warned Japanese Prime Minister Shinzo Abe not to revise the country's apology over the forced sexual enslavement during World War II and called for efforts to resolve its wartime atrocities.

Historians say up to 200,000 women, euphemistically called "comfort women" and mostly from Korea, were forced into sexual slavery for front-line Japanese soldiers.

Resisting consistent calls for extending a formal apology and providing compensation to the victims, Abe and his Liberal Democratic Party have campaigned to revise an apology the country's then-Chief Cabinet Secretary Yohei Kono issued in 1993 for the forced sex slavery of women during Japanese colonization, contending that Japan did not coerce the women.

In a letter to Abe, Rep. Mike Honda and Rep. Steve Israel voiced "serious concern" about the Abe government's stance in dealing with the issue and urged it to come forward to resolve the long-standing grievance.

"It would have grave implications for the U.S.-Japanese relationship, and could ignite unnecessary tension and provocation with neighboring countries (if Japan revises the apology)," the two Democratic lawmakers said in a letter that was delivered to Japanese Ambassador to Washington Kenichiro Sasae on Wednesday.

The letter comes two days before the Japanese premier is scheduled to visit Washington for summit talks with his U.S. counterpart.

"Japan's government must fully acknowledge, apologize for and increase awareness of its history of comfort women," Rep. Israel said in a statement issued on Thursday, stressing Japan's need to express "sincere regret for a systematic atrocity in order to move forward as a democracy."

Pointing out that the government is a "living, breathing organism that is responsible for its past, present and future," Rep. Honda also said "nothing is more important (now than for Japan) to formally acknowledge and unequivocally apologize for its systematic atrocity."

The matter is one of the most emotional pending issues between South Korea and Japan, which ruled the Korean peninsula between 1910-45. Seoul has repeatedly stressed that the issue be resolved urgently, as most victims are elderly and may die before they can receive compensation or an apology from Tokyo.

Japan, however, has ignored Seoul's calls for action, claiming the matter was settled by a 1965 treaty that normalized relations between the two countries. 

"The grandmothers -- those survivors of physical, sexual and psychological violence perpetuated by Japan's Imperial Army – are still waiting for an appropriate apology," said Rep. Honda.

In 2007, the House of Representatives unanimously approved a resolution, spearheaded by Honda, calling on Japan to take responsibility for its forced sexual enslavement of women.

The two lawmakers planned to present a second resolution on the issue, according to experts on the matter. (Yonhap News)