The skulls, bones, and names of thousands of dead at the Nanjing massacre memorial stand as a stark demonstration of China and Japan's inability to move beyond history in their increasingly tense relationship.

Six million people a year view the skeletal remains of victims -- displayed where they fell -- of a six-week spree of killing, rape and destruction after the Japanese military entered China's then-capital on December 13, 1937.

Chinese lawmakers on Thursday made the anniversary of the massacre an official day of remembrance, along with September 3 to mark the country's victory against Japan in 1945.

The declarations were just Beijing's latest gestures in a diplomatic battle as a territorial dispute festers between the two Asian powers.

A group of nearly 40 Chinese citizens filed a lawsuit at a Beijing court on Wednesday demanding compensation from two Japanese companies, including Mitsubishi Materials Corp., for forced wartime labour.

Articles and commentaries critical of Japan are a near-daily mainstay in Chinese state media, and Beijing's foreign ministry regularly denounces Tokyo at its daily briefings.

"Japan's right-wing forces... have become troublemakers undermining regional peace and stability," spokeswoman Hua Chunying said Wednesday, when seven of the 13 questions asked concerned the neighbour in some way.

History still bears heavy on the relationship, most weightily in Nanjing.

China says 300,000 people died in the eastern city, although some respected foreign academics put the number lower. China historian Jonathan Spence estimates that 42,000 soldiers and citizens were killed and 20,000 women raped, many of whom later died.

Wang Yang was among three generations of her family, including her 13-year-old daughter, viewing the gruesome photos in the memorial hall.

"She's a little young, but she needs to understand this history," she said of her child.

- 'History must not be forgotten' -

The Nanjing memorial -- where entry is free -- has drawn comparisons to the Auschwitz concentration camp of Nazi Germany and the Hiroshima memorial for Japanese victims of the US atomic bomb.

But the authorities have also included explicitly political messages.

"Under the inspiration of patriotic enthusiasm, we should struggle unceasingly for the construction of socialism with Chinese characteristics," reads one sign.

It describes the site as a "spiritual treasure to draw historical lessons so as to sustain the goals of peace and development", but adds that it is "an important topic for the patriotic education of the common people, especially youth".

A young man touring the memorial with friends said simply: "I feel hate."

The director of the memorial, Zhu Chengshan, insisted it was not intended to whip up hostility towards the Japanese.

"We are not seeking hate, it's for historical education. We also have a theme of peace," he said.

Chinese people take issue with Japan's failure to adequately apologise for wartime atrocities and the denial by some that a massacre took place, he said.

Ties between the Asian giants turned for the worse in 2012, after Japan nationalised islands in the East China Sea it administers but which are claimed by both sides, igniting street protests across China which the normally strict authorities allowed.

In November last year, Beijing imposed an air defence identification zone (ADIZ) over the islands, which it calls Diaoyu and Tokyo refers to as Senkaku, saying it required notification from planes crossing the area.

Japanese Prime Minister Shinzo Abe then made a controversial visit to the Yasukuni Shrine, which honours 14 top war criminals from World War II among the country's war dead, in December, sparking more anger from China.

The Chinese government earlier this month hosted more than 40 foreign journalists at the Nanjing memorial, granting access to a massacre survivor, so they could "see with their own eyes" evidence of Japanese atrocities, a foreign ministry spokeswoman said.

Viewing a photo of the decapitated head of a Chinese with a cigarette stuck in its mouth, supposedly as a joke by a Japanese soldier, a woman visitor said: "If Chinese people come to Nanjing, they definitely have to come here first. I feel bitter."

Some ultra-conservative Japanese politicians dispute that the atrocities occurred.

But the Japanese government points to 1995 and 2005 statements by the then-prime ministers, both of which used the phrase "heartfelt apology".

On Nanjing, Tokyo says that "the killing of a large number of noncombatants, looting and other acts occurred", and adds "it is difficult to determine" the correct number of victims.

Nanjing memorial guide Xu Jingjing believes the site can serve as a reminder. "History must not be forgotten. That is the philosophy and goal of building this museum," she said.