In this Feb. 2, 2017 file photo, Secretary of State Rex Tillerson, accompanied by accompanied by State Department Undersecretary for Political Affairs Tom Shannon, left, takes the podium to speak to State Department employees upon arrival at the State Department in Washington. Shannon told agency staffers on Thursday that he will retire as soon as a successor for his Senate-confirmed post is chosen and ready to assume the job. (AP Photo/Andrew Harnik)

In this Feb. 2, 2017 file photo, Secretary of State Rex Tillerson, accompanied by accompanied by State Department Undersecretary for Political Affairs Tom Shannon, left, takes the podium to speak to State Department employees upon arrival at the State Department in Washington. Shannon told agency staffers on Thursday that he will retire as soon as a successor for his Senate-confirmed post is chosen and ready to assume the job. (AP Photo/Andrew Harnik)

WASHINGTON (AP) — The Trump administration is losing its top career diplomat in a heavy blow to the State Department amid numerous international crises.

The retirement of the department’s third-ranking official, Undersecretary of State for Political Affairs Tom Shannon, was announced on Thursday at a time when the administration’s foreign policy and its treatment of veteran diplomats has come under heavy criticism.

While not a household name, Shannon is widely respected by his colleagues, lawmakers and others, and his departure in the coming months will leave another void in the top ranks. He holds the rank of “career ambassador” — the highest in the foreign service.

Shannon informed State Department staffers that he will retire as soon as a successor is chosen and ready to assume the job after receiving Senate confirmation. He is a near 35-year veteran of the U.S. Foreign Service and was the most senior department official to remain in his job after the transition from the Obama to the Trump administrations. He has served under six presidents of both parties since 1984 and 10 secretaries of state and was seen as a symbol of diplomatic continuity and professionalism during last year’s tumultuous transition.

Shannon’s departure is sure to be seized on by critics of the administration who accuse Trump and Secretary of State Rex Tillerson of gutting the foreign service, particularly its senior ranks. Of five career ambassadors on the job when Tillerson arrived at the State Department a year ago Thursday, only one remains and that diplomat is currently on sabbatical. Of six undersecretary positions, only two, including Shannon’s, are currently occupied. The rest are vacant.

Sen. Bob Menendez of New Jersey noted in a tweet there is just one remaining career ambassador and said, “I fear poor management at State that has led to the loss of staff, and failure of the Trump Admin. to nominate enough people to fill vacant positions is risking international security.”

In an interview, Shannon said he was retiring for personal and not political reasons.

“I decided it was an important moment to take a step back and to determine what next I can do in my life,” he said. He said the death of his mother last year and his own 60th birthday last week contributed to his decision. But he also said he considered himself the “designated survivor” after former President Barack Obama tapped him for the undersecretary post in February 2016 with the expectation that he would bridge the gap between administrations whoever was elected that November.

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“Aside from helping the Obama administration to the finish line, I knew my job was also about helping this institution navigate the political transition, get across the river,” Shannon said. “And, once across that river, help this institution — the foreign service and civil service — be responsive to our elected leadership.”

Tillerson, who has been criticized for his leadership of the State Department and for removing or forcing out senior career officials, said he had asked Shannon to stay on and acknowledged that his departure would be a loss.

“He is a walking encyclopedia,” said Tillerson, who dropped by during the interview in Shannon’s office just down the hall from his own on what is known as “Mahogany Row” on the State Department’s seventh floor. “Thirty-five years of experience is not something you replace overnight,” he said.

Tillerson said Shannon was welcome to return if he did not enjoy retirement. “There will always be a place for Tom Shannon at the State Department,” Tillerson said.

Reaction to Shannon’s announcement was quick and effusive with a number of his current and former foreign service colleagues wishing him well and thanking him for his decades of service.

Former Secretaries of State Hillary Clinton and John Kerry both weighed in.

“Congrats on a well-deserved retirement for Tom Shannon,“Kerry said in a tweet. “We are all — all ten secretaries he worked with — better off for the gift of his service.”

“Tom has served our country with excellence and dedication in senior posts under Democratic and Republican administrations alike, continuing in the Foreign Service’s proud tradition of nonpartisan expertise and professionalism,” Clinton wrote in a statement.

The union that represents U.S. diplomats, the American Foreign Service Association, saluted Shannon “for his exemplary 35-year diplomatic career and outstanding service to our nation. He will be missed.”

The chairman of the Senate Foreign Relations Committee, Sen. Bob Corker, tweeted that Shannon has served the department and “our country honorably for 35 years. I have enjoyed working with him and join Secretary Tillerson in thanking him for a job well done.”

Shannon, who has extensive diplomatic experience in Africa and Latin America and led U.S. delegations to Russia last year, said he had confidence in younger foreign service officers to carry on despite today’s “hyper-politicized” atmosphere in Washington. Some 60 percent of foreign service officers have been diplomats for less than 10 years.

“It would be my hope that, independent of the political forces that swirl in this town, that especially my foreign service colleagues, both present and past, will recognize the importance of honoring the profession and our ethos of service,” he said.