"The comfort women issue has become a global issue, not a bilateral one," the spokesman quoted Chung as saying.

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Chief Cabinet Secretary Yoshihide Suga, the government's top spokesman, also said Japan "sent a strong message" to Seoul.

Chung Hyun-Back, the gender equality minister, hit back through her spokesman, saying the government will push through with the document registration as well as with a project she announced Monday to build a museum in memory of the women.

Toru Hashimoto, the young, brash mayor of Osaka who is also co-leader of an emerging conservative political party, also told reporters that there was not clear evidence that the Japanese military coerced women to become what are euphemistically called "comfort women".

"To maintain discipline in the military, it must have been necessary at that time," said Hashimoto.

"For soldiers who risked their lives in circumstances where bullets are flying around like rain and wind, if you want them to get some rest, a comfort women system was necessary.

That's clear to anyone." Historians say up to 200,000 women, mainly from the Korean Peninsula and China, were forced to provide sex for Japanese soldiers in military brothels.

The panel, which consists of 18 independent international experts, has been taking and evaluating two batches of Japan's replies since it offered its concluding observations in July 2014.

The paper criticised the country's denial of the forcible nature of its sexual enslavement of Korean women during World War II, though it entailed the state's "direct legal responsibility" while carrying concerns over the "revictimisation" of former the euphemistically labelled comfort women by some public officials.

Washington is keen for it not to hamper cooperation against North Korea's nuclear and missile programmes.