One of Japan's rising political stars has sparked controversy by saying the use of wartime sex slaves was necessary for brave soldiers risking their lives.

It is believed thousands of women were forced to serve as Japanese army "comfort women" during World War II.

Around three quarters of comfort women died, while many survivors were left scarred and infertile.

Speaking to reporters, Osaka mayor Toru Hashimoto said Japan needed to reflect on and apologise for the massive pain it inflicted during the war.

But he went on to say that the use of comfort women was necessary for brave soldiers who were risking their lives.

"In the circumstances in which bullets are flying like rain and wind, the soldiers are running around at the risk of losing their lives," he is reported as saying.

"If you want them to have a rest in such a situation, a comfort women system is necessary. Anyone can understand that."

He says other armies also had similar systems.

Mr Hashimoto asked why the world only questions Japan's use of comfort women during the war, complaining that his country is being insulted.

He acknowledged that comfort women served soldiers against their will.

In most cases comfort women were kidnapped or lured away from their homes in Japanese-occupied territories.

They were then imprisoned in "comfort stations" around the world and the women who survived the ordeal have told of daily rapes and beatings at the hands of Japanese soldiers.

The term comfort women is translated from the Japanese word ianfu, which is a euphemism for the word prostitute.