US President Donald Trump's new picks for secretary of state and CIA chief could face tough confirmation hearings before the Senate, though some have welcomed the biggest White House shakeup since Trump assumed office last January.

Trump fired Rex Tillerson via tweet Tuesday morning, saying his replacement would be CIA Director Mike Pompeo.

Gina Haspel, Pompeo's deputy at the CIA, would be the first woman to lead the agency. Both nominees face a confirmation hearing by the Senate.

Senate Majority Leader Mitch McConnell urged the US Senate to act quickly to confirm the nominees.

"Both of these nominees seem to be well-qualified and ... we hope they'll not be subjected to undue delay, which has been an enormous problem," Reuters cited McConnell as saying on Tuesday.

But the president's Republican Party has a slim majority with 51 seats in the Senate, meaning confirmation is not certain.

If confirmed by the Senate to replace Tillerson, Pompeo could breathe new vigor into an agency all too often sidelined on many of the nation's most pressing national security matters under Tillerson, The Associated Press reported.

Pompeo, reportedly more in sync with Trump than Tillerson, will face a trying test upon his arrival in Foggy Bottom: how he will play a role in putting together a Trump-Kim summit agreed to last week.

Senator Ron Wyden of Oregon, a Senate Intelligence Committee member, said he opposed the nominations of both Pompeo and Haspel to their new positions.

"Before and after his confirmation as CIA director, Mike Pompeo has demonstrated a casual relationship to truth and principle," he said in a statement on Tuesday.

But Senate Minority Leader Chuck Schumer said on his Twitter account on Tuesday 'that he hopes Pompeo will "turn over a new leaf" as secretary of state.

As deputy director of the CIA, Haspel had not previously undergone Senate onfirmation. A career veteran at the CIA, she oversaw a secret agency prison where terror suspects were subjected to a harsh interrogation technique, the AP reported on Tuesday.

Senator John McCain said Haspel needs to explain the "nature and extent" of her' involvement in the CIA's interrogation program. He called the torture of detainees in US custody "one of the darkest chapters in American history" and he said the Senate must scrutinize her involvement in the "disgraceful program", according to the AP.

Trump told reporters he has worked closely with Haspel and considers her "an outstanding person", Reuters reported.

Tillerson said his department "exceeded the expectations of almost everyone" on the Democratic People's Republic of Korea. He also listed the areas where much work remains to be done, including forging a way forward with China.

Tillerson said he had delegated his official duties to Deputy Secretary of State John Sullivan, and his own tenure would end on March 31.

Jessica Chen Weiss, professor of government at Cornell University and an expert on Chinese foreign policy, said so far there's little indication that Trump consistently listens to his advisers, while there's every indication that his advisors follow Trump's lead.

"Pompeo fits this profile as well and has little diplomatic experience, so leaders and Asia and around the world will still have to guess from Trump's tweets and spur-of-themoment decisions what new tack the US plans to take," she ' said in a statement on Tuesday.

"China and Asia can probably expect more of Trump's wild swings between rhetorical confrontation and cooperation," she said.

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