Amb. Barbara Stephenson, president of the American Foreign Service Association (AFSA), a union for US foreign service personnel, wrote the letter for the December issue of the group's publication.
"While I do my best, as principal advocate for our institution and as a seasoned American diplomat, to model responsible, civil discourse," she wrote, "there is simply no denying the warning signs that point to mounting threats to our institution -- and to the global leadership that depends on us."
CNN obtained an advance copy of the letter
, which was distributed to AFSA members on Tuesday.
"There is no denying that our leadership ranks are being depleted at a dizzying speed, due in part to the decision to slash promotion numbers by more than half," she wrote, noting 60 percent of career ambassadors have left the State Department since January, with similarly dramatic attrition at the next most senior levels.
Career ambassadors are the most elite US diplomats in the foreign service. Fewer than 60 individuals have attained the rank since it was instituted in 1955. Of the five people who held the rank at the beginning of the administration, three have since retired.
In her letter, Stephenson lamented the department's partial hiring freeze, which has slowed the intake of junior foreign service officers.
"As the shape and extent of the staffing cuts to the Foreign Service at State become clearer," Stephenson wrote, "I believe we must shine a light on these disturbing trends and ask 'why?' and 'to what end?'"
Stephenson did not single out Secretary of State Rex Tillerson or President Donald Trump in the letter, but said that "the need to make the case for the Foreign Service with fellow Americans and our elected representatives has taken on a new urgency."
The State Department is attributing the recent dip to its ongoing reorganization effort.
"The goal of the redesign has always been to find new ways to best leverage our team's brains, ingenuity and commitment to serving our nation's interests," a State Department official told CNN in a statement. "AFSA and other employee groups are important partners in the redesign effort. As has been said many times before, the freezes on hiring, promotions, are only temporary while we study how to refine our organization."
However, the official added, "suggestions that drastic cuts to our foreign service ranks are taking place are simply not accurate."
The overall number of diplomats serving the State Department is similar to last year when individuals pending congressional confirmation are included, the official noted, and there has been a decline in prospective new foreign officers taking the entrance exam.
"Secretary Tillerson remains fully committed to the success of our team, the professional development of the foreign service and America's global leadership," the official added. "These facts will again be reinforced once the freezes on personnel movement, including hiring, are lifted at the appropriate time."