"I am pleased to announce that I intend to nominate highly respected Admiral Ronny L. Jackson, MD, as the new Secretary of Veterans Affairs," said Trump, who has also replaced his secretary of state and national security advisor in the last month.

"I am thankful for Dr. David Shulkin's service to our country and to our GREAT VETERANS!" he said on Twitter.

Pentagon official Robert Wilkie would serve as interim department chief until Jackson receives Congressional approval, the president added.

Shulkin's sacking had been widely predicted after he was accused of spending $122,000 on a nine-day trip to Europe with his wife, which included sightseeing at castles and taking in professional tennis matches.

He was one of the few senior officials in the Trump administration who had also served during the presidency of Barack Obama.

Jackson's stock with Trump rose after he pronounced in January that the 71-year-old president was in "excellent" health.

After releasing the results from Trump's physical, Jackson said "he has incredible genes and that's the way God made him."

VoteVets, a progressive organization, urged Senators to "ask him blunt, direct questions about how the VA works" during his confirmation hearing.

"We are concerned about his readiness to assume control," the group wrote on Twitter.

AMVETS Executive Director Joe Chenelly questioned whether Jackson, who has no experience in management, is qualified to run a $200 billion a year bureaucracy, the second largest agency in the US government.

"With an official bio that does not seem to contain any indication that he's held a command, is the president's nominee fully prepared to lead such a massive bureaucracy?," Chenelly asked in a statement.

- 'Hate to see him go' -

Shulkin's departure caps a dizzying month of changes to the Trump cabinet, most notably the sacking of Rex Tillerson as secretary of state on March 13.

Trump's top economic advisor Gary Cohn also exited the White House after the president announced plans to impose tariffs on imports of steel and aluminum, a policy Cohn did not endorse.

And National Security Advisor HR McMaster was axed only six days ago, replaced by the hardline Fox News pundit and former UN ambassador John Bolton.

The Department of Veterans Affairs is the largest federal agency after the Pentagon, employing 360,000 people.

Lack of funding has weakened the hospital network specifically dedicated to the health of veterans -- a key constituency for Trump.

And the system has come under criticism from many veterans and organizations for falling short, particularly in terms of psychiatric care.

Hundreds of thousands of veterans, many who served in Iraq and Afghanistan, suffer from post-traumatic stress disorder.

The 58-year-old Shulkin had served as undersecretary of veterans affairs for health under Obama.

He previously worked as chief medical officer of the University of Pennsylvania health system.

During his tenure as secretary, he oversaw legislative changes including an expansion of benefits for post-9/11 veterans.

Following Trump's announcement, Phil Roe, a Tennessee Republican who chairs the House Committee on Veterans Affairs, praised Shulkin for a "fantastic job," adding: "I hate to see him go."

"That said, I respect President Trump's decision, support the president's agenda and remain willing to work with anyone committed to doing the right thing on behalf of our nation's veterans."