BEIJING — China is warning online video sites to clean up their acts, raising the prospect of state restrictions on content in the fast-growing medium that until now has been self-regulating.

The State Internet Information Office and the State Administration of Radio, Film and Television (SARFT) issued a joint circular asking online video content providers “to closely examine their videos before making them available for viewing.”

The watchdog said “good” web skeins and films could help develop a positive web culture.

“The intention is to build a healthy environment for online programming but some online programs contain seriously vulgar and violent content. A few shows even use lewd and gory scenes as a publicity stunt, stirring up controversy on the Internet,” a spokesperson told the China Daily.

To date, online video sites effectively censor themselves and don’t go through the usual channels.

China’s online video market has more than 400 million viewers and is fast emerging as the future of TV in China, where many people access content on their handsets.

A report by JP Morgan analyst Dick Wei predicts that China’s online video market will be worth $4.4 billion by 2015, accounting for 20% of the online ad market and 6% of the total ad market.

Earlier this week, Chinese online TV company Youku inked a licensing deal with NBCUniversal’s international TV distribution arm for VOD rights for a selection of current, library and new content on Youku Premium, which has more than 300 million users.