Chinese Premier Li Keqiang said Thursday that Beijing has "zero tolerance" for corruption however senior the culprit, as the country's leaders press forward with a much-publicised anti-graft campaign.

But he did not address reports that one of the ruling Chinese Communist Party's former top officials may be under investigation, telling his once-a-year press conference only that "everybody is equal before the law".

"We will show zero tolerance to those corrupt behaviours and corrupt officials," Li said at the close of the annual session of the National People's Congress (NPC), China's rubber-stamp legislature.

"China is a country that is under the rule of law," he continued.

"No matter who he is and how senior his position is, if he violates party discipline and law of the country, he will be seriously dealt with and punished to the full extent of the law, because everybody is equal before the law."

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

Neither Li nor any of the selected questioners mentioned Zhou Yongkang, the former domestic security czar and member of the Politburo Standing Committee (PSC), China's most powerful body, who is widely rumoured to be under investigation.

If the Zhou probe is confirmed it would be the first time in decades that such a high-ranking figure has been targeted in a formal inquiry, and would send shockwaves through China's elite.

PSC members have generally been regarded as untouchable even after retirement.

News outlets were warned by the Chinese Foreign Ministry not to ask questions about Zhou at the press conference or "they would face being blacklisted from asking questions in the future", Hong Kong's South China Morning Post reported Thursday.

- 'Cheese will be moved' -

In his remarks on corruption, Li made apparent reference to the motivational book, "Who Moved My Cheese?", a US best-seller that counsels businesspeople on how to adapt to change.

"In the course of reform, the vested interests will be shaken, and some people's cheese will be moved, so to speak," Li said.

"For example, in the course of power delegation, some government departments will find fewer powers in their own hands, and in boosting market competition and easing market access, some existing companies will feel greater pressure."

Li also addressed China's environmental challenges, returning to his rhetoric last week that China would "declare war" on pollution.

"To declare a war against smog doesn't mean that we are going to declare a war against nature," Li said.

"Rather, what we mean is that we are going to declare a war against our own inefficient and unsustainable model of growth and way of life."

He added that "those overseeing agencies that turn a blind eye to polluting activities and fail to perform their duties of overseeing will be held accountable".

As China faces rising tensions with its Asian neighbours -- particularly Japan -- over a series of maritime disputes, Li asserted Beijing's "unshakeable will" in safeguarding its territorial sovereignty.

"When neighbours interact with each other, it's only natural that sometimes they will run into problems of one kind or another," he said.

"But as long as they respect each other, properly manage differences and pursue mutual benefit, there will be harmonious sounds instead of jarring noises."