Donald Trump fires Homeland Security Secretary Kristi Noem

Kristi Noem has been criticised for her handling of ICE raids. (AP: Alex Brandon)

US President Donald Trump has fired Secretary of Homeland Security Kristi Noem after mounting criticism over her leadership of the department.

Mr Trump made the announcement on social media on Thursday, local time, two days after Ms Noem faced a grilling on Capitol Hill from Republicans and Democrats.

Ms Noem, who Mr Trump said "served us well," will take over a new role called "Special Envoy for the Shield of the Americas".

It is "a new Security Initiative in the Western Hemisphere we are announcing on Saturday," Mr Trump wrote on Truth Social.

"I thank Kristi for her service at 'Homeland."

Ms Noem has been under pressure over her handling of immigration enforcement operations.

The secretary described two civilians shot dead by ICE agents in separate incidents in Minnesota as engaged in acts of domestic terrorism. Video evidence did not back up those claims.

The killing of Renee Good and Alex Pretti caused significant public backlash against the president's immigration agenda.

Another point of scrutiny was a US$220 million ad campaign featuring Ms Noem that encouraged people in the country illegally to leave voluntarily.

Ms Noem previously told lawmakers that Mr Trump was aware of the campaign in advance, but the US president disputed that in an interview with Reuters on Thursday, saying he did not sign off on the ad campaign.

Aside from immigration, Ms Noem also faced criticism over the pace of emergency funding approved through the Federal Emergency Management Agency and for the Trump administration's response to disasters.

Ms Noem is the first cabinet secretary to leave during Mr Trump's second term.

Mr Trump says Oklahoma senator Markwayne Mullin will replace her.

Markwayne Mullin says he has "no idea" how quickly his nomination will move. (AP Photo: Jacquelyn Martin)

"A MAGA Warrior, and former undefeated professional MMA fighter, Markwayne truly gets along well with people," Mr Trump said.

"Markwayne will work tirelessly to Keep our Border Secure, Stop Migrant Crime, Murderers, and other Criminals from illegally entering our Country, End Scourge of Illegal Drugs and, MAKE AMERICAN SAFE AGAIN."

Mr Mullin would need to be confirmed by the Senate. But under a federal law governing executive branch vacancies, he would be allowed to serve as an acting Homeland Security secretary as long as his nomination is formally pending.

Voting in the Senate just after Mr Trump's announcement, Mr Mullin said he had "no idea" how quickly his nomination would move.

"The president and I are good friends. So we look forward to working closer with the White House, and obviously I'm gonna be over there a lot more," he said.

ABC/AP