U.S. President Donald Trump fired Veterans Affairs Secretary David Shulkin by tweet Wednesday in the wake of a bruising ethics scandal and a mounting rebellion within the agency, and nominated White House doctor Ronny Jackson to lead the agency.

A Navy rear admiral, Jackson is a surprise choice to succeed Shulkin, a former Barack Obama administration official and the first non-veteran to head the department. Trump had been considering replacements for Shulkin for weeks, but had not been known to be considering Jackson for the role.

In a statement, Trump praised Jackson as "highly trained and qualified."

....In the interim, Hon. Robert Wilkie of DOD will serve as Acting Secretary. I am thankful for Dr. David Shulkin’s service to our country and to our GREAT VETERANS! —@realDonaldTrump

Jackson has served since 2013 as the physician to the president, and gained a national profile earlier this year for holding a sweeping press conference on the president's health.

The promotion of Jackson marks the latest Trump hire to be driven at least as much by personal familiarity with the president as by his vision for the role.

Brig.-Gen. Dr. Richard Tubb, who trained Jackson, said in a letter read at Jackson's briefing that the doctor had been attached like "Velcro" to Trump since Inauguration Day.

"On any given day," he wrote,"the 'physician's office,' as it is known, is generally the first and last to see the president."

A White House official said Shulkin was informed of his dismissal by Chief of Staff John Kelly before the president announced the move on Twitter on Wednesday afternoon.

Veterans' organization concerns

Trump had considered several others for the post, including conservative Fox & Friends contributor Pete Hegseth. The White House was hopeful Jackson will have a smoother confirmation process because he was chosen for his current position during the Obama administration.

But a major veterans' organization expressed concern over Shulkin's dismissal and Trump's intention to nominate Jackson, whom they worried lacked experience to run the huge department.

"We are disappointed and already quite concerned about this nominee," said Joe Chenelly, the national executive director of AMVETS. "The administration needs to be ready to prove that he's qualified to run such a massive agency, a $200 billion bureaucracy."

Rep. Phil Roe, chairman of the House Veterans Affairs Committee, said he believed Shulkin did a "fantastic job" and didn't think he should have been dismissed, but "at the end of the day, cabinet secretaries serve at the pleasure of the president."

"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," said Roe, a Republican from Tennessee.

"I am in the process of reaching out to Dr. Jackson and I look forward to building a strong relationship with him also."

Shulkin is the second cabinet secretary to depart over controversies involving expensive travel, following former Health and Human Services secretary Tom Price's resignation last September.

Trump plans to nominate White House physician Dr. Ronny Jackson as Veterans Affairs Secretary. (The Associated Press)

Trump has selected Robert Wilkie, the undersecretary of Defence for Personnel and Readiness, to serve as the acting head of Veterans Affairs. It is government's second largest department, responsible for nine million military veterans in more than 1,700 government-run health facilities. The selection of Wilkie bypasses Deputy Secretary Tom Bowman, who has come under criticism for being too moderate to push Trump's agenda of fixing veterans' care.

Shulkin had insisted he had the full confidence of the White House amid continuing investigations over his travel and leadership of the department. He had agreed to reimburse the government more than $4,000 after Veterans Affairs internal watchdog concluded last month that he had improperly accepted Wimbledon tennis tickets and his then-chief of staff had doctored emails to justify his wife travelling to Europe with him at taxpayer expense.

Internal department drama

Shulkin also blamed internal drama at the agency on a half-dozen or so political appointees who were rebelling against him and Bowman, insisting he had White House backing to fire them.

But the continuing infighting and a fresh raft of watchdog reports documenting leadership failures and spending waste — as well as fresh allegations being reviewed by the IG that Shulkin used a member of his security detail to run personal errands — proved too much of a distraction.

It was the latest in a series of departures for top administration officials, including Secretary of State Rex Tillerson, who was fired by Trump earlier this month.

The sudden departure comes as Trump is currently seeking to expand the Veterans Choice program, a campaign promise that major veterans' groups worry could be an unwanted step toward privatizing Veterans Affairs health care. His plan remains in limbo in Congress after lawmakers declined last week to include it in a spending bill.

Having pushed through legislation in Trump's first year making it easier to fire bad Veterans Affairs employees and speed disability appeals, Shulkin leaves behind a department in disarray. Several projects remain unfinished, including a multibillion-dollar overhaul of electronic medical records aimed at speeding up wait times for veterans seeking medical care as well as expanded mental health treatment for veterans at higher risk of suicide.

During the presidential campaign, Trump repeatedly pledged to fix the department, which was still reeling after a 2014 scandal at the Phoenix VA medical centre, in which veterans waited months for care even as Veterans Affairs employees created secret waiting lists to cover up delays. Criticizing the department as "the most corrupt," Trump said he would bring accountability and expand access to private doctors, promising to triple the number of veterans "seeing the doctor of their choice."

Currently, more than 30 per cent of Veterans Affairs appointments are made in the private sector.