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

Mr 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," Mr Li said. "The progress we have made still falls short of the expectations of our people."

One year ago, Mr 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 Mr 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.

Mr Li was asked about the video on 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", Mr Li said, adding that the enforcement of China's environmental law must not be "soft".

Mr Li also said that Beijing will "resolutely" protect citizens near its south-western border, after a bomb dropped by a Myanmar warplane killed five people on Chinese territory.

China has sent fighter jets to patrol the border after the incident, in which a bomb hit a sugarcane field in Lincang in Yunnan province on Friday, killing five workers and injuring eight others.

It came days after Beijing warned of escalating violence near the border following a surge in ethnic conflict in the remote Kokang region in Myanmar's northeastern Shan state.

Beijing was a key backer of Myanmar's military junta while it was under Western sanctions, but President Thein Sein has increased ties with other countries including the United States since launching political reforms in 2011.

Mr Li expressed "condolences and deep sympathies to the victims and their families" at the meeting.

"At the same time, our government, the ministry of foreign affairs and the military have made solemn representations to the Myanmar side," he said.

"We have the responsibility and the ability to resolutely safeguard the security and stability of the China-Myanmar border, and we will resolutely protect the lives and property of our people," he said.

China's People's Liberation Army Air Force on Friday sent several fighter jets to "track, monitor, warn and chase away" Myanmar military planes flying close to the border, air force spokesman Shen Jinke was quoted by China's official news agency Xinhua as saying.

It cited Fan Changlong, vice-chairman of the Central Military Commission, as telling the commander of Myanmar's military that Beijing will take "firm and decisive action" in the event of any repetition.

Chinese Deputy Foreign Minister Liu Zhenmin summoned the Myanmar ambassador to Beijing, Thit Linn Ohn, on Friday night to protest against the deaths, the agency added.

Last month, Myanmar declared a state of emergency in Kokang in response to the conflict, which began on Feb 9.

The unrest has virtually emptied the main Kokang town of Laukkai, the epicentre of the fighting, with streets in the once-bustling frontier community transformed into a battleground.

More than 30,000 people have fled from Myanmar into Yunnan province, according to Xinhua.