With its red-painted window frames and sooty facade, the empty two-storey building on Gongping Road is typical of many of Shanghai’s pre-second world war structures – but this one houses dark memories.

The crumbling mansion is one of 150 sites in the city formerly used as “comfort stations”, part of the vast system of sexual slavery established by Japan for its invading armed forces before and during the war. About 30 are believed left in the city, but these silent witnesses to history are disappearing amid rapid urban development and China’s hesitance to memorialise the painful episode.

“All these historical remains are slowly being demolished. There are fewer and fewer,” said Bao Xiaqin, an expert on China-Japan relations at the city’s Fudan University.

Mainstream historians agree that about 200,000 “comfort women”, mostly from Korea but also from other Asian nations including China, were forced to work in Japanese military brothels during the war.

Calls to preserve such sites, and remember their victims’ suffering, have until recently been muzzled by China’s desire to play down one of the most sensitive issues in its stormy relationship with Japan.

There are glimmers of hope. The Gongping Road site was to be razed in a redevelopment plan but was saved last year after historian Su Zhiliang highlighted the building’s past and Chinese media amplified it. But it remains an uphill battle for Su, of Shanghai Normal University, who has waged a crusade to spotlight the suffering of sex slaves.

When he first began delving into the issue in the early 1990s, authorities prevented Su from publishing his research.

“The Chinese government has really not done enough. This is a wartime human rights issue, but in order to maintain good relations with Japan the government does not give the issue much support,” Su said.

He now raises money for survivors, of whom there are only 17 known in China, none in Shanghai. Many were stigmatised and ostracised after the war, receiving no special government assistance.

But a rightward shift in Japanese politics under Prime Minister Shinzo Abe, and a more assertive China under President Xi Jinping, are giving oxygen to the issue, experts say.

A former “comfort station” in Nanjing, 300km west of Shanghai, was converted into a museum by local authorities, opening in December 2015. Su, meanwhile, was granted permission to upgrade his display of archives into a museum, which opened in October in a building on his campus.

Just outside, a statue representing two sex slaves – one Chinese, one Korean – was unveiled.

China has also recently made available documents on sex slaves from its official archives, amid an international effort to include them in the Unesco International Memory of The World Register.