A classified advert paying tribute to victims of the 1989 Tiananmen Square crackdown has slipped past censors to appear in a Chinese newspaper.

Chinese authorities are reportedly investigating how the advert came to appear on page 14 of the Chengdu Evening News on Monday, the 18th anniversary of the crackdown.

Comment on the events in 1989 is still taboo in China with the government calling the incident a student-led "counter-revolutionary" or subversive riot.

The one-line advert in the Chengdu Evening News reportedly read: "Paying tribute to the strong(-willed) mothers of June 4 victims".

According to Hong Kong's South China Morning Post newspaper the advert was passed by a young clerk who had never heard of the 1989 incident.

The Post said the young woman, who was not named, had recently graduated and worked for an advertising company responsible for receiving content for the ads section.

"She called the man back two days later to check what June 4 meant and the man said it was (a date on which) a mining disaster took place," the Post quoted a source at the paper as saying.

"This highlights (the fact) that the government needs to face up to history," the paper quoted the source as saying.

A staff member in the editor's office at the Chengdu Evening News told Reuters the source of the advert was "under investigation" but gave no further comment.

The agency also quoted sources close to the paper saying authorities were questioning newspaper staff to try to find out who placed the advertisement in the classified section and if any staff member collaborated by turning a blind eye to it.

One source who requested anonymity said the authorities had identified the person who placed the ad as a local activist.

Human rights groups say hundreds, possibly thousands of people were killed in the military crackdown after the government ordered the armed forces to clear Tiananmen Square of protestors on the night of June 4.

Many of the leaders of the Tiananmen protest fled into exile or remain locked up in Chinese labour camps.

References to the crackdown are barred in state media, and other printed works while websites on the subject are blocked meaning large portions of many China's younger generation have little or no idea of the events of 1989

Source: Agencies