The US National Security Agency (NSA) has denied allegations by Fox News host Tucker Carlson that it is spying on him in an attempt to force him off air.

On Monday Carlson, a long-time critic of President Joe Biden, said a government whistle-blower had told him the NSA was monitoring him for "political reasons".

The NSA denied this, saying its primary role is to counter foreign threats.

Fox News has not commented publicly on its host's claim.

In response to a question about Carlson's comments, White House spokesperson Jen Psaki said the NSA "focuses on foreign threats and individuals who are attempting to do us harm on foreign soil."

"Beyond that I would point you to the intelligence community," she added.

The NSA, together with the Central Security Service, is responsible for intercepting and protecting electronic communications on behalf of the US government, and has a broad cyber security remit.

During a segment of Tucker Carlson Tonight - one of America's most popular news programmes - Mr Tucker said he was convinced by the alleged whistle-blower because they had information about an ongoing story "that could have only come directly from my texts and emails."

He accused the agency of "planning to leak them in an attempt to take this show off the air", and said Fox News had filed a freedom of information request "for all information that the NSA and other agencies have gathered about this show."

In response, the NSA issued a statement on Twitter denying the accusation, adding that Tucker Carlson had never been an intelligence target of the agency.

"With limited exceptions (e.g. an emergency), NSA may not target a US citizen without a court order that explicitly authorizes the targeting," it said.

But on his show on Tuesday, Carlson said this was "infuriatingly dishonest" and attacked the White House for failing to issue a denial.

He recalled several unsuccessful attempts to speak on the phone with the NSA's director, General Paul Nakasone, who he said was "highly political" and "left-wing".

Gen Nakasone was appointed by Republican President Donald Trump in 2018.

"Did the Biden administration read my emails? NSA officials refuse to say," said Carlson. "In a very heated follow-up conversation 20 minutes ago, they refused even to explain why they won't answer that simple question."

The NSA's work came under scrutiny in recent years after its surveillance of US telephone records was exposed by former contractor Edward Snowden in 2013. Last year, the US Court of Appeals ruled that the programme was illegal.

Carlson has also come under the spotlight. Earlier this year, senior US military leaders condemned him for criticising measures to support women in the armed forces.

The Fox show host was also criticised in 2019, after audio recordings emerged of him defending child marriage and making sexual comments about underage girls.