US Air Force Secretary Heather Wilson, who was considered a top candidate to become the next secretary of defence, has decided to resign after almost two years to return to academia, US officials told Reuters, leaving another vacant post at the top level of the Pentagon.
Ms Wilson was not asked to leave by anyone in President Donald Trump's administration and was not resigning under pressure, the official told Reuters.
Former Fox News executive Bill Shine has also resigned as White House communications director to join Mr Trumps re-election campaign as a senior adviser.
White House press secretary Sarah Sanders says that Trump accepted Shine's resignation on Thursday evening. The resignation was effective on Friday.
"It has been a privilege to serve alongside our Airmen over the past two years and I am proud of the progress that we have made in restoring our nation's defences," Ms Wilson, 58, said in her resignation letter, a copy of which was seen by Reuters.
She plans to step down on May 31 to become president of the University of Texas at El Paso, said the official, who spoke on condition of anonymity.
The University of Texas Board of Regents still has to approve Wilson's selection but she is the sole finalist.
The resignation leaves another senior Pentagon job open and follows the December departure of Defense Secretary Jim Mattis, who quit over policy differences with Mr Trump and had picked Ms Wilson for the job.
Mr Mattis' deputy, Patrick Shanahan, is performing the role in an acting capacity in what is widely seen as an audition for the job. Ms Wilson's resignation could add to speculation that Mr Shanahan may remain in the position.
A Pentagon spokesman told a news conference that he could not confirm Ms Wilson's resignation but that he had no reason to doubt the Reuters story.
"Everyone she has talked to wants her to stay but she thinks the time is right to take on this new challenge," the US official said, speaking on condition of anonymity.
Wilson informed Vice President Mike Pence of her decision earlier in the week and Air Force Chief of Staff David Goldfein on Thursday, the official said.
It is unclear who might succeed Ms Wilson and inherit steep challenges facing the Air Force, which include the creation of Trump's "Space Force," a new branch of military service that will carve out some responsibilities current done by the Air Force. The Air Force is also reeling from a fresh scandal involving sexual assault.
Ms Wilson was the first Air Force Academy graduate to ever take the highest position in her service, and counted a robust resume that included a decade as a Republican lawmaker in Congress. She also served on the National Security Council staff during the George HW Bush administration, and as president of the South Dakota School of Mines and Technology.
After joining the Pentagon, Ms Wilson visited Iraq and Afghanistan and came away concerned about the wear-and-tear on an Air Force that she thought was too small, especially as the Pentagon shifted its focus to competition with Russia and China.
Last fall, she predicted the Air Force would need to grow sharply over the next decade or so, boosting the number of operational squadrons by nearly a quarter to stay ahead of Moscow and Beijing.
She told reporters at the time that the preliminary analysis drew partly from classified intelligence about possible future threats, showing that Air Force, at its current size, would be unable to preserve America's edge.
Ms Wilson estimated the Air Force would need about more 40,000 personnel as part of the plan to have a total of 386 operational squadrons, compared with 312 today. The U.S. Air Force had 401 squadrons in 1987, at the peak of the Cold War.