South Korea and Japan are in behind-the-scenes talks to resolve tensions over Tokyo's wartime sexual slavery in a way where Tokyo issues a new statement of apology in exchange for Seoul's promise not to litigate the issue again, according to a U.S. news report.

The Washington Post also reported in a column that the two countries are trying to reach a compromise before South Korean President Park Geun-hye's visit to Washington later this month or at least by June 22, the 50th anniversary of the normalization of Japan-South Korea relations.

"Intensive back-channel negotiations have been taking place," David Ignatius said in the column. "What would it take to make the past less toxic? U.S. and South Korean officials envision a similar process. Japan would offer a new statement of contrition about the comfort women, going beyond Abe's expression of remorse for 'human trafficking,'"

U.S. officials thought the phrase avoided the issue of who did the "trafficking," he said.

"Officials speculate that (Japanese Prime Minister Shinzo) Abe might make a personal gesture, such as writing a letter to comfort women. In return, Japan would get assurance that this issue won't be 'relitigated' endlessly," he said.

A senior Japanese official was quoted as saying, "I believe efforts have been made to make progress, but (they have) not yet produced the final result."

The column also quoted a South Korean official agreeing that "progress is being made, but we are not there yet."

"History issues are a serious challenge for now, but that doesn't stop us from developing relationships (with Japan) at other levels," the official was quoted as saying.

Historians estimate that up to 200,000 women, mainly from Korea, which was a Japanese colony from 1910 to 1945, were forced to work in front-line brothels for Japanese soldiers during World War II. But Japan has long attempted to water down the atrocity.

Abe came under strong fire for failing to issue a clear apology for the atrocity during his visit to the United States in April that included a landmark address at a joint meeting of Congress.

The sexual slavery issue has been the biggest thorn in frayed relations between Japan and South Korea, with Seoul demanding Tokyo take steps to address the grievances of elderly Korean victims of the atrocity and Japan refusing to do so. (Yonhap)