China on Tuesday expressed shock and indignation at a Japanese politician's statement that the Japanese military's World War II-era sex slaves served a "necessary" role by keeping troops in check.

Foreign Ministry spokesman Hong Lei said the conscription of the sex slaves, known as "comfort women", was a grave crime committed by the Japanese military and a major human rights issue that concerns the victims' personal dignity.

The remarks by Osaka Mayor Toru Hashimoto flagrantly challenge historical justice and the conscience of humankind, Hong said at a regular news conference.

"How Japan treats its past will decide its future," he said, adding that the country's neighbors, as well as the international community, will have to wait to see what choice Japan makes.

Analysts said several prominent Japanese politicians' recent remarks that fly in the face of historical facts are a result of the rise of Japan's extreme nationalism and its troubled domestic economic situation.

"To maintain discipline in the military, it must have been necessary at that time," said Hashimoto, also the co-leader of the national Japan Restoration Party.

"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."

He also said that there isn't clear evidence that the Japanese military coerced women into sexual slavery.

Zhou Yongsheng, an expert on Japanese studies at China Foreign Affairs University, said it is an undeniable historical fact that the Japanese military forced a large number of women, including those from China, into sexual slavery for Japanese soldiers during World War II.

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