A group of nearly 200 academics, including Pulitzer Prize winners, has published an open letter calling on Japan to face up to its World War II crimes, including its system of sex slavery.

The letter, penned by scholars from top institutions including Harvard, Yale, the University of Chicago and the London School of Economics, comes as disquiet grows over what critics say is the tendency of Japan's Prime Minister Shinzo Abe to whitewash the past.

"This year presents an opportunity for the government of Japan to show leadership by addressing Japan's history of colonial rule and wartime aggression in both words and action," the letter says.

The missive, which was published on the Internet and is not addressed to anyone in particular, says Japan has achieved great things in the 70 years since its surrender.

But it says an apparent refusal by some on the right to fully accept Tokyo's guilt, risked undermining that stance.

The scholars argue that even by the standards of wartime sexual violence and military prostitution in the last century, Japan's so-called comfort women system "was distinguished by its large scale and systematic management under the military".

It also noted the system's "exploitation of young, poor, and vulnerable women in areas colonized or occupied by Japan."

Mainstream historians say around 200,000 women, mostly from Korea but also from other Asian nations, were systematically raped by Japan's imperial forces in military brothels.

Japanese conservatives, however, say no official documents prove government involvement in the system; they say the women were common prostitutes engaged in a commercial exchange.

They have also argued that memories of the survivors cannot be trusted and are highly politicised in an issue that serves as one of the main geopolitical fault lines running through East Asia.

Signatories of the letter include John W. Dower, professor emeritus of history at Massachusetts Institute of Technology, whose book "Embracing Defeat" masterfully tells the story of Japan's rise from the ashes of WWII.

They also include Herbert Bix, professor emeritus of history and sociology at Binghamton University and author of the acclaimed biography "Hirohito and the Making of Modern Japan", which examined the life and influence of the wartime emperor.

Both works won Pulitzer Prizes and are required reading for any student of Japan.

The letter comes as Abe readies a formal statement to mark the 70th anniversary of the end of hostilities. All eyes are on whether he will repeat previous explicit prime ministerial apologies for Japanese violence.