A hard-hitting video investigation into China's grave air pollution problem has been pulled from mainstream video sites, days after it garnered more than 150 million hits online.

"Under the Dome", an independent documentary produced by former Chinese state media journalist Chai Jing, was no longer available on popular mainland video sites, including Youku and iQiyi, as of Saturday afternoon.

A link on Youku's website that previously led to the video now prompts the message: "We're very sorry, Youku was unable to find the page you requested."

The 103-minute documentary -- hailed by some as China's "Inconvenient Truth" -- remains available on YouTube, which is blocked in China.

Versions of the video had racked up more than 155 million views on mainland Chinese video streaming sites just one day after its release last Saturday.

In the video, Chai, who previously worked as an anchor for state-run China Central Television, detailed causes of atmospheric pollution in the country, including slack government supervision and lenient penalties for polluters.

She has described the video as her "personal battle" against air pollution after her daughter was born with a benign tumour.

The removal of the documentary underscores the ruling Communist Party's sensitivity to public debate over China's notorious smog problem.

It also represents a sharp turnaround by Chinese authorities, who only days ago encouraged ubiquitous coverage of the video in official print and broadcast media.

China's newly-appointed environmental protection minister, Chen Jining, praised the video earlier this week, telling Chinese reporters that it should "encourage efforts by individuals to improve air quality".

The video's disappearance comes as the country's top annual political meeting, the National People's Congress (NPC), is underway in Beijing, under thick white skies and with the city's air quality registering as "very unhealthy", according to a US embassy reading.

Online discussions related to the video remain unblocked on China's popular social networks, and users of China's Twitter-like Sina Weibo on Saturday voiced frustration with the government's abrupt censorship move.

"Chai Jing's documentary, 'Under the Dome', has already been 'harmonised' on all of the mainstream video sites," wrote one user on Sina Weibo, using an ironic term for authorities' blocking of objectionable content. "Why? Give us a reason first!"

"When will this country be able to face the attitudes of its own people?" another Sina Weibo user wrote.

- 'Iron hand' -

China's cities are often hit by heavy pollution, blamed on coal-burning by power stations and industry, as well as vehicle use, and it has become a major source of popular discontent with the Communist Party, leading the government to declare a "war on pollution" and vow to reduce the proportion of energy derived from fossil fuels.

Chen, the environmental protection minister, Chen held an hour-long news conference Saturday afternoon on the sidelines of the NPC.

But out of a dozen questions asked -- mostly by Chinese state-run media outlets -- not a single one focused on the documentary.

NPC press conferences are usually highly stage-managed by Chinese authorities, with organisers pre-approving both the order of questions as well as the questions themselves.

Chen did make a reference to "APEC blue" -- a tongue-in-cheek phrase that became popular online during the Asia-Pacific Economic Cooperation summit in November, when Beijing shut down factories and enacted strict regulations to ensure a blue sky for visiting dignitaries.

"From international experience and the process of bringing about 'APEC blue', we can see that if we want to significantly improve our air quality, we cannot rely on heaven alone; we must bring down our emissions levels," Chen said.

"Can it be done?" he asked. "It can, but it's very difficult indeed, and it will require us all to make an extra effort."

Swarmed by international and Chinese journalists after the news conference, he ignored reporters' questions as he quickly exited the hall.

Even as the video was being deleted from major video sites, China's top leader, President Xi Jinping, was sounding a tough line on pollution.

"We are going to punish, with an iron hand, any violators who destroy ecology or the environment, with no exceptions," Xi told a group of lawmakers Friday, according to the official Xinhua news agency.