SEOUL (Bloomberg) - South Korean President Park Geun Hye said Japanese Prime Minister Shinzo Abe missed an opportunity to improve ties between their countries by not apologising for Japan's wartime use of sex slaves during his United States trip.

"There is much criticism even in the US about the Abe government not taking advantage of the chance to strengthen trust with neighbouring nations with a sincere apology over historical issues including the 'comfort women' victims," Ms Park said on Monday at a meeting with secretaries, according to the website of her office.

Ms Park's demands that Mr Abe do more to atone for Japan's wartime aggression have added to tensions over a territorial dispute and soured Korean-Japaneses relations. The rift between the two-biggest US allies in North Asia is complicating President Barack Obama's shift to Asia and his effort to maintain a united front to contend with a nuclear North Korea and a more assertive China.

Ms Park has demanded Mr Abe do more to apologise to the Korean victims before she will agree to a two-way summit. More than 50 former comfort women remain alive in South Korea, and have become a symbol of Korean suffering during Japan's colonial rule from 1910 until the end of World War II in 1945.

During his speech at the US Congress last week, Mr Abe expressed "deep remorse" for unspecified suffering caused by his nation's wartime actions while offering no new apology to the women from Korea and other nations forced into sexual slavery by Japan's Imperial Army in the early 20th century.

Earlier in the trip, Mr Abe said he was "deeply pained to think about the comfort women who experienced immeasurable pain and suffering as a result of victimization due to human trafficking".

Mr Abe has been lobbying to improve ties with Ms Park and also China, which shares South Korea's concerns about historical issues.

He told Ms Park after the funeral for Singapore's founder Lee Kuan Yew in March that he hopes for a summit with China and South Korea.