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South Korea's president has apologized to Korean women forced into sexual slavery by Japan's imperial army in World War II over what he calls a flawed 2015 deal with Tokyo to settle disputes over the issue.

President Moon Jae-in issued the apology during a meeting with former sexual slavery victims on Thursday. It's the first time that Moon has apologized over the deal, which was struck before he took office last May.

A state-appointed panel recently concluded Seoul's previous conservative government failed to properly communicate with the victims before reaching the deal with Japan. Moon later ordered officials to map out measures to meet the victims' demands.

Tokyo warned any attempt to revise the accord would make bilateral relations "unmanageable" and "unacceptable."