The United States is losing its top career diplomat and the State Department's third-highest-ranking official with the retirement of Thomas Shannon.
Shannon, the undersecretary of state for political affairs, announced Thursday that he was calling it a career after nearly 35 years spent serving six presidents.
He is the fourth career diplomat to leave the State Department since Donald Trump became president a year ago.
"My decision is personal, and driven by a desire to attend to my family, take stock of my life, and set a new direction for my remaining years," Shannon said in a statement. He turned 60 last week.
Secretary of State Rex Tillerson said someone of Shannon's experience and knowledge cannot be "replaced overnight."
State Department spokeswoman Heather Nauert called Shannon "a patriot, a diplomat and a great American."
Shannon was acting secretary of state between the end of the Obama administration and Tillerson's confirmation.
Shannon came to the State Department in 1984 under then-President Ronald Reagan. He has served both Republican and Democratic administrations in numerous posts, including in Brazil, Venezuela, Cameroon, Gabon and South Africa.
He has agreed to stay on the job until the Senate confirms his successor.
Shannon's resignation is sure to bring on more criticism of the Trump administration and what Democrats say is Tillerson's gutting of the diplomatic service in his zeal to downsize the State Department and reduce bureaucracy.
Senator Bob Menendez, a New Jersey Democrat, noted in a tweet that Shannon's departure would leave just one career diplomat on the job.
"I fear poor management at State that has led to the loss of staff and failure of the Trump administration to nominate enough people to fill vacant positions is risking international security," Menendez wrote.
Thirteen assistant secretary and undersecretary of state positions and dozens of ambassadorships remain unfilled.
Critics say budget and staff cuts have created low morale within the State Department.