Premier Li Keqiang of China said on Sunday that the government was failing to satisfy public demands to stanch pollution and would impose heavier punishments to cut the toxic smog that was the subject of a popular documentary belatedly banned by censors.

The premier’s news conference at the end of the annual full meeting of the National People’s Congress has become a fixture of the Chinese political calendar, cast as a show of political candor and accountability. But the briefings have mostly become a stilted ritual, with questions generally preselected and massaged to avoid the airing of controversies about legal rights, corruption scandals and other themes unwelcome by Communist Party leaders.

This year’s conference was no different. But Mr. Li took one reporter’s question about air pollution, which mentioned the banned documentary, “Under the Dome,” and he acknowledged that there was a gap between the government’s efforts and public ire about pervasive smog.

“This is a concern that is uppermost on all people’s minds,” Mr. Li said in response to a question from a Huffington Post reporter, who asked about the government’s struggle to clean up the environment.

“The Chinese government is determined to tackle smog and environmental pollution as a whole,” Mr. Li said. “But the progress we have made still falls far short of the expectation of the people. Last year, I said the Chinese government would declare war against environmental pollution. We’re determined to carry forward our efforts until we achieve our goal.”

The sky over Beijing was pale blue on Sunday, though a brownish haze smudged the horizon, and the capital’s air was rated “unhealthy” by a United States Embassy air monitor. And even with clearer skies during some of the legislative session, pollution has loomed as an issue.

“Under the Dome,” an online exposé made by a famous former state television reporter, went viral before the start of the session on March 5. On the second day of the National People’s Congress meeting, officials ordered websites to delete the film.

While China’s pollution may not be as dire as urban India’s, the reaction to “Under the Dome” showed that many Chinese people were irate that the mere act of breathing could be a health hazard. The soup of particles and gases from vehicles, industry and power plants that often bathes Beijing and other Chinese cities testifies to the failure of agencies and local governments to act on the government’s promises of cleaner air. Last year, according to the Ministry of Environmental Protection, eight of 74 monitored cities met clean air standards.

Mr. Li pointedly made no mention of “Under the Dome” and its banning. But he acknowledged some of the problems raised by the documentary, especially lax enforcement of pollution restrictions by environmental agencies too weak to take on state energy conglomerates. Mr. Li said the government would fully enforce the newly amended environmental protection legislation.

“All acts of illegal production and emissions will be brought to justice and held accountable,” Mr. Li said. “We need to make businesses that illicitly emit and dump pay a price too heavy to bear. We must ensure that the enforcement of the environmental protection law is not a stick of cotton candy but a powerful mace.”

Though a reporter had asked about it, Mr. Li avoided directly blaming state-owned enterprises, including the big oil companies, that many environmental advocates say are blocking policies and stifling the enforcement of regulations designed to protect the environment. But Mr. Li said the government would work to fully implement the Environmental Pollution Law, which allows for greater fines against polluters and which environmental advocates had praised when details were announced months ago.

The slower economic target of around 7 percent growth this year, which was announced by Mr. Li and endorsed by the legislature, could help the government reduce pollution, by reducing demand for coal.

Mr. Li has made similar remarks on various issues — the environment, official corruption and the economy — three years in a row now, ever since holding the first of these scripted news conferences in March 2013 after taking the post of prime minister.

On Sunday he issued targets for reducing carbon dioxide intensity — the amount of the greenhouse gas emitted for each unit of economic activity — by 3.1 percent, and he said the government would introduce legislation for a long-discussed “environmental protection tax.”

Last year, China’s coal consumption fell 2.9 percent, according to government statistics. Glen Peters, a scientist at the Center for International Climate and Environmental Research-Oslo, has estimated that the drop in consumption, together with slowed growth in cement production, reduced China’s annual emissions of carbon dioxide, the main greenhouse gas from human activity, 0.8 percent. This was the first fall in China’s emissions after more than 15 years of fast growth, he said.

Discussion of public anger about the pollution was, however, mostly muted throughout the National People’s Congress. The Communist Party-controlled legislature has 2,965 delegates, overwhelmingly handpicked party members and officials, who meet in full session once a year to discuss and, inevitably, approve the government’s plans. Even so, some of the public frustration crept into the meeting.

One delegate, Zhu Lieyu, a lawyer, has proposed that the environment minister deliver an annual report to the legislature, as the premier does, and resign if delegates fail to approve the report.

The environment minister, Chen Jining, who took office in recent weeks, has also acknowledged the government’s failings.

“In the past, it was normal not to enforce environmental laws,” Mr. Chen said at a news conference in Beijing after becoming minister. “Now we have to turn that around so that abiding by the law is normal.”

Right after the online release of “Under the Dome,” Mr. Chen praised it as akin to “Silent Spring,” the landmark 1962 book that energized the environmental movement in the United States. But on March 7, one day after censors banned the documentary, Mr. Chen avoided any mention of the film at what appeared to be a carefully scripted news conference.