SEOUL, April 16 -- Senior diplomats of South Korea and Japan started talks Wednesday about Japan's wartime sexual enslavement of Korean women ahead of U.S. President Barack Obama' s visit to the two countries next week.

Junichi Ihara, head of Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau, arrived in Seoul to meet with Lee Sang- deok, director general of South Korean Foreign Ministry's Northeast Asian Affairs Bureau.

It was the first time South Korea and Japan held talks about the comfort women, a euphemism for Korean women coerced into sexual slavery for the Japanese Imperial Army during World War Two.

The diplomats reportedly planned to hold a marathon dialogue and a dinner session, as differences on the comfort women issue remained between the two countries.

Advocate groups for the former sex slaves have demanded that the Japanese government acknowledge its legal responsibility for the wartime crime.

In the 1990s, Japan set up the so-called Asian Women's Fund, a private-public fund to compensate for former sex slaves in Asia, but many South Korean victims refused to take money as it did not directly come from the Japanese government.

Japan has claimed all issues related to the wartime atrocities, including the comfort women issue, were resolved under the 1965 treaty that normalized the diplomatic relations between the two nations.

Among 237 South Korean women who identified themselves as former sex slaves, only 55 are still alive.

The United States has pressed its two close Asian allies to mend ties ahead of Obama's visit to Japan and South Korea. Obama is scheduled to arrive in Seoul on April 25 for a two-day visit after staying in Tokyo for three days.

South Korean President Park Geun-hye has refused to meet one-on- one with Japanese Prime Minister Shinzo Abe since her inauguration in February last year as ties between Seoul and Tokyo have been strained since Abe returned to power in December 2012 due to Abe' s wrong perception of history and territorial dispute over the islets, known as Dokdo in South Korea and Takeshima in Japan, lying halfway between the two nations.

The relations worsened after Abe paid a visit in December last year to the controversial Yasukuni Shrine, which honors 14 convicted Class-A war criminals.