Veteran US diplomat William Burns, nominated to lead the CIA, pledged Wednesday to keep the agency free of politics and said China would be his biggest focus if confirmed.

Burns was picked by President Joe Biden to replace retiring Central Intelligence Agency Director Gina Haspel and to help restore an air of independence after former president Donald Trump allegedly tried to manipulate the country's spies for political reasons.

He told the Senate Intelligence Committee that his career in the Middle East and Russia made him appreciate the value of untainted intelligence.

"I learned that good intelligence, delivered with honesty and integrity, is America's first line of defense," he told the panel.

"I learned that intelligence professionals have to tell policymakers what they need to hear, even if they don't want to hear it. And I learned that politics must stop where intelligence work begins."

He said Biden stressed that "he wants the agency to give it to him straight."

"And I pledged to do just that, and to defend those who do the same."

Burns also told the panel that China was the main challenge the country faces, echoing views stated by Biden and other top officials of his administration.

"Out-competing China will be key to our national security in the decades ahead. That will require a long-term, clear-eyed, bipartisan strategy, underpinned by domestic renewal and solid intelligence," he said.

He said that while in some areas, like nuclear non-proliferation, the United States can cooperate with Beijing, it is nevertheless "a formidable, authoritarian adversary."

The CIA in particular has faced challenges, with China having recruited a number of US diplomats and spies over recent years and also, starting a decade ago, bringing down the CIA's own network of informants in the country.

Burns also stressed in a statement to the hearing that the fight for technological superiority in espionage, including the use of artificial intelligence, will also be a key part of his mission.

Until his nomination Burns was president of the Carnegie Endowment for International Peace, a leading Washington foreign policy think-tank.

If approved as expected by the Senate, Burns will be the first career diplomat to lead the CIA.

But he has deep experience in security and intelligence matters after spending over three decades in the US foreign service, including a stint as ambassador to Russia from 2005-2008.