China is falling short of its people's expectations in battling smog, Premier Li Keqiang said Sunday, one week after authorities blocked a scathing documentary on the country's air pollution problem.

Li, who is second only to President Xi Jinping in the Communist hierarchy, made the comments at his once-a-year meeting with journalists at Beijing's Great Hall of the People.

Reporters attending the annual event are typically required to submit their questions in advance.

"The Chinese government is determined to tackle smog and pollution," Li said. "The progress we have made still falls short of the expectations of our people."

One year ago, Li pledged that China would declare "war on pollution", and the premier said Sunday that Beijing is "determined to continue our efforts".

"We will continue to track down and pursue polluting companies," he said, adding that "environmental authorities must not suffer interference from industry, and should be brave enough to take on responsibility".

China's cities are often hit by heavy pollution, blamed on coal-burning by power stations and industry, as well as vehicle use.

The issue has become a major source of popular discontent with the Communist Party, leading the government to vow to reduce the proportion of energy derived from fossil fuels.

Last month, former Chinese state media journalist Chai Jing released a hard-hitting independent documentary on the country's air pollution problem.

The 103-minute video investigation, "Under the Dome", racked up more than 155 million views on mainland Chinese video streaming sites just one day after its release.

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

But days later the documentary was taken down from China's mainstream video sites, and Chen did not address it in an hour-long news conference on the sidelines of the National People's Congress (NPC), China's rubber-stamp legislature.

Li was asked about the video Sunday and the role of China's state-owned oil companies in pollution.

He did not directly address the film issue but in a reference to China's annual top political meetings said: "Since environmental protection has been continuously stressed in the Two Sessions, the people running companies should not be so silly to overlook it.

"They must have already noticed it. But why do they continue to ignore the policies made by the state?" he asked.

Illegal emitters -- "no matter what type of company they are" -- will "pay a price too high to bear", Li said, adding that the enforcement of China's environmental law must not be "soft".