The government has expressed its displeasure to U.S. officials over a plan to unveil in California next week a monument dedicated to Korean "comfort women" forced into sexual slavery by the Japanese military during the war, according to diplomatic sources.

The plan by a U.S.-South Korean civic group to erect the monument on a public lot at a park in Glendale goes against Tokyo's views on the comfort women issue and it has urged local officials to take appropriate measures, the sources said.

Japan has repeatedly claimed the issue was settled by a Japan-South Korea treaty in 1965 that normalized diplomatic ties and has provided compensation through a private fund. But the former comfort women have continued to call for official compensation.