China has approved for release the first film with gay principal characters, the movie's director said, in what some are calling a breakthrough in the country's heavily-censored media.

Director Wang Chao announced on Weibo, a Chinese version of Twitter, that censors had given "Seek McCartney" permission for a cinema release.

"This is a small step for the film department, and a big step for the members of the film industry," Wang posted on Saturday.

China only decriminalised homosexuality in 1997, but conservative attitudes remain widespread and discrimination is common, although tolerance is greater in major cities.

Those who come out to friends and family in China often face significant pressure to undergo supposed sexuality "treatment" or marry a partner of the opposite sex.

The film, a Chinese-French co-production, centres on a secret relationship between two men, one Chinese and one French.

But some warned caution, given the Chinese government's unpredictable implementation of censorship.

"The fact that this film can be released in theatres doesn't mean gay films in the future will be able to released in China," Fan Popo, an LGBT filmmaker and rights activist, told AFP on Wednesday.

"China's system for evaluating films is still very unstable, because the rules are very unclear. It depends heavily on the individual censor's whims."

Fan added that, while gay characters have appeared in supporting roles in Chinese films, "Seek McCartney" is the first time the plot centres on a homosexual couple.

Gay rights campaigners in China still face harassment from authorities or bureaucratic roadblocks.

LGBT groups in China are barred from registering as official non-governmental organisations, and activists often take a low-profile approach to promoting events in case the authorities decide to crack down.