The US has condemned recent remarks by Osaka Mayor Toru Hashimoto justifying Japan's war-time use of sex slaves as "outrageous and offensive".
A US state department spokesman said such women had been victims of a "grave human rights violation".
Mr Hashimoto said on Monday that the use of such women had been "necessary".
Japan forced an estimated 200,000 women in occupied territories to become prostitutes for troops during World War II.
Many of the women came from China and South Korea, but also from the Philippines, Indonesia and Taiwan.
The Japanese government, which in 1993 issued a formal apology over the issue, has sought to distance itself from his remarks.
"Mayor Hashimoto's comments were outrageous and offensive," said state department spokeswoman Jen Psaki.
"What happened in that era to these women who were trafficked for sexual purposes is deplorable and clearly a grave human rights violation of enormous proportions," she said.
Mr Hashimoto, the co-founder of the nationalist Japanese Restoration Party, which has a small presence in parliament, said enforced prostitution had been necessary to keep troops in line.
"If you want them [troops fighting a war] to have a rest in such a situation, a comfort women system is necessary. Anyone can understand that."
He also provoked outrage on the island of Okinawa - home to a large US military presence - by suggesting that base-related crime could be reduced if US troops were encouraged to use the local sex industry so that "the sexual energy of those tough guys" could be controlled.
On Thursday Mr Hashimoto offered to meet former sex slaves and "apologise firmly" for Japan's actions, calling them "a disgraceful act" that should not be repeated.
But he reiterated his stance that Japan was not alone in exploiting local women during wartime.
"Everybody was doing bad things. I think Japanese people... should offer objections if there is a misunderstanding of facts in the world."
And on Okinawa, he blamed a cultural gap with the US on perceptions of prostitution, saying his "international sensitivity was quite poor when I had to operate beyond national borders".
Both South Korea and China have condemned Mr Hashimoto's remarks, which come at a time when historical issues and territorial rows have elevated tensions in the region.
A new right-wing government led by Prime Minister Shinzo Abe took power in December 2012.
Last month, he angered China and South Korea when he suggested he may no longer stand by the wording of Japan's 1995 apology for its war-time aggression, saying the definition of "aggression" was hard to establish.
Japanese ministers later sought to play down his remarks, amid anger across the region - where the feeling that Japan has not sufficiently atoned for, or educated its population enough about, its war-time behaviour is deeply entrenched.