The comments made Monday are already raising ire in neighboring countries that bore the brunt of Japan's wartime aggression and have long complained that Japan has failed to fully atone for wartime atrocities.
Toru Hashimoto, the young, brash mayor of Osaka who is co-leader of an emerging conservative political party, also said that U.S. troops currently based in southern Japan should patronize the local sex industry more to help reduce rapes and other assaults.
Hashimoto told reporters on Monday that there wasn't clear evidence that the Japanese military had coerced women to become what are euphemistically called "comfort women" before and during World War II.
"To maintain discipline in the military, it must have been necessary at that time," Hashimoto said. "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.
China's Foreign Ministry criticized the mayor's comments and saw them as further evidence of a rightward drift in Japanese politics under Prime Minister Shinzo Abe.
"We are appalled and indignant about the Japanese politician's comments boldly challenging humanity and historical justice," Foreign Ministry spokesman Hong Lei said at a daily media briefing. "The way they treat the past will determine the way Japan walks toward the future. On what choice Japan will make, the Asian neighbors and the international community will wait and see."
Asked about a photo of Abe posing in a fighter jet with the number 731 - the number of a notorious, secret Japanese unit that performed chemical and biological experiments on Chinese in World War II - Hong again urged Japan not to whitewash history so as to improve relations with countries that suffered under Japanese occupation.
"There is a mountain of definitive iron-hard evidence for the crimes they committed in the Second World War. We hope Japan will face and contemplate their history of aggression and treat it correctly," Hong said.
Abe posed, thumbs up, in the aircraft during a weekend visit to northeastern Japan.
South Korea's Foreign Ministry expressed disappointment over what it called a senior Japanese official's serious lack of historical understanding and respect for women's rights. It asked Japan's leaders to reflect on their country's imperial past, including grave human rights violations, and correct anachronistic historical views.
Hashimoto said he recently visited Okinawa in southern Japan and told the U.S. commander there "to make better use of the sex industry."
"He froze, and then with a wry smile said that is off-limits for the U.S. military," he said.
"I told him that there are problems because of such formalities," Hashimoto said, explaining that he was not referring to illegal prostitution but to places operating within the law. "If you don't make use of those places you cannot properly control the sexual energy of those tough guys."
Calls to the after-hours number for U.S. Forces in Japan were not answered.
Hashimoto's comments came amid continuing criticism of Abe's earlier pledges to revise Japan's past apologies for wartime atrocities. Before he took office in December, Abe had advocated revising a 1993 statement by then Prime Minister Yohei Kono acknowledging and expressing remorse for the suffering caused to the sexual slaves of Japanese troops.
Abe has acknowledged "comfort women" existed but has denied they were coerced into prostitution, citing a lack of official evidence.
Recently, top officials in Abe's government have appeared to backpedal on suggestions the government might revise those apologies, apparently hoping to ease tensions with South Korea and China and address U.S. concerns about Abe's nationalist agenda.
Chief Cabinet Secretary Yoshihide Suga repeated the previous government position and said those women went through unbearable pain.
"The stance of the Japanese government on the comfort women issue is well known. They have suffered unspeakably painful experiences. The Abe Cabinet has the same sentiments as past Cabinets," he said.
Education Minister Hakubun Shimomura said Hashimoto's remark was unhelpful given the criticism Japan faces from neighboring countries and the U.S. over its interpretation of history.
"A series of remarks related to our interpretation of (wartime) history have been already misunderstood. In that sense, Mr. Hashimoto's remark came at a bad time," Shimomura told reporters. "I wonder if there is any positive meaning to intentionally make such remarks at this particular moment."
Hashimoto, 43, is co-head of the newly formed Japan Restoration Party with former Tokyo Gov. Shintaro Ishihara, who is a strident nationalist.
Sakihito Ozawa, the party's parliamentary affairs chairman, said he believed Hashimoto's remarks reflected his personal views, but he expressed concerns about possible repercussions.
"We should ask his real intentions and stop this at some point," he said.