Story highlights AFSA leader said 60% of career ambassadors have left the State Department since January

CNN obtained an advance copy of the letter distributed to AFSA members on Tuesday

(CNN) The head of an organization representing US diplomats around the world excoriated the State Department's leadership in a letter Wednesday, saying the agency's senior tiers "are being depleted at a dizzying speed," resulting in "a decapitation of its leadership ranks."

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.

Read More