"Japan - reveal the truth! Admit the crime! Officially apologise! Punish the criminals!" South Korean protesters chant every Wednesday outside the Japanese Embassy in Seoul.

They are the survivors of the brutal, Asia-wide system of sex slaves for the Imperial Japanese Army, which the military government encouraged and helped to operate for 13 years, from 1932 until the end of World War II in 1945.

They were euphemistically called "comfort women". But experts like Korean American scholar Edward Chang of the University of California say the network of "comfort stations" were actually officially-sanctioned rape camps.

Many of the women were even killed as part of an attempt to cover up the crime.

"There should be no time limit on prosecuting these crimes against humanity," Prof Chang said.

Japan says all potential claims by individuals for sufferings inflicted in the war were closed years ago, by treaties normalising its ties with other Asian countries.

But Kang Kyung-wha, a senior official at South Korea's foreign ministry, has recently urged Japan to come to terms with its "legal responsibility" and human rights obligations towards the former comfort women.