United States Secretary of State Rex Tillerson will be replaced by Central Intelligence Agency (CIA) Director Mike Pompeo, US President Donald Trump tweeted on Tuesday morning.

Pompeo will be replaced at the CIA by Gina Haspel, his deputy. She will be the first woman in that role.

"Mike Pompeo, director of the CIA, will become our new secretary of state. He will do a fantastic job! Thank you to Rex Tillerson for his service! Gina Haspel will become the new director of the CIA, and the first woman so chosen. Congratulations to all!" the president tweeted.

The US president offered no explanation for the change.

Tillerson did not speak to Trump before he was sacked Tuesday and has not been given a reason for his ouster, a top aide said.

“The secretary did not speak to the president this morning and is unaware of the reason, but he is grateful for the opportunity to serve, and still believes strongly that public service is a noble calling and not to be regretted,” Undersecretary of State Steve Goldstein said.

Tillerson had arrived back in Washington before dawn on Tuesday after a tour of African countries and a few hours later Trump announced abruptly on Twitter that he was to be replaced by CIA director Mike Pompeo.

“We wish Secretary-Designate Pompeo well,” Goldstein — who is undersecretary of state for public affairs and public diplomacy — said, in a series of tweets from his official account.

“The secretary had every intention of remaining because of the tangible progress made on critical national security issues. He established and enjoyed relationships with his counterparts,” he said.

“The secretary will miss his colleagues at the Department of State and enjoyed working together with the Department of Defense in an uncommonly robust relationship,” he added, in a nod to Tillerson's close working relationship with US Secretary of Defense Jim Mattis.