Reuters was first to report
on the memo, and obtained other documents appearing to support the argument that several bureaus within the State Department recommended the three countries be listed.
CNN has not independently obtained the memo, but a senior State Department official familiar with the document confirmed its authenticity. It was sent to Brian Hook -- a top aide to Secretary of State Rex Tillerson -- through an internal dissent channel within the department.
In the memo, the employees object to the exclusion of Afghanistan, Iraq and Myanmar from the list, "despite the clearance of the Office of Legal Adviser and relevant regional and functional bureaus that there were sufficient facts demonstrating that all three governments either had governmental armed forces or supported armed groups that recruited and used child soldiers in 2016 and therefore met the legal requirements for a listing."
"Beyond contravening U.S. law," they add, "this decision risks marring the credibility of a broad range of State Department reports and analyses and has weakened one of the U.S. government's primary diplomatic tools to deter governmental armed forces and government-supported armed groups from recruiting and using children in combat and support roles around the world."
Asked at a photo opportunity why his agency decided to leave the countries off the list, Tillerson hesitated, then simply said, "It's all in the report."
The secretary of state is required by law to annually identify countries that use or support the use of child soldiers, and does so within the agency's regular Trafficking in Persons Report.
The President of the United States can issue waivers to offending countries if doing so "is in the national interest," and must alert Congress within 45 days.
"No one in the United States government likes the idea of child soldiers," State Department spokeswoman Heather Nauert said at a briefing Tuesday. "It's abhorrent."
However, she said Tillerson had reviewed all the evidence when deciding what countries to put on the offenders' list in June and did not agree that Iraq, Afghanistan and Myanmar merited inclusion.
"He made his own decision on this," she acknowledged, "but it was not without reviewing all the information that came from the various bureaus."
In a letter to Tillerson on Friday, the top Democrat on the Senate Foreign Relations Committee, Sen. Ben Cardin of Maryland, expressed concern over the content of the employees' memo, and "the precedent this action may set and the message it could send to countries that still have not ended child soldiering."
Listing a country on the offenders' list can hinder the United States' ability to provide military support to that country.
In this year's report, the State Department ultimately singled out eight countries: The Democratic Republic of Congo, Mali, Nigeria, Somalia, South Sudan, Sudan, Syria and Yemen.
In 2016, Iraq and Myanmar (sometimes identified as Burma) were on the list
, but Afghanistan was not.
In their dissent memo, the employees also took issue with the arguments offered publicly and internally by department leadership for excluding the three countries, and said these were not legally relevant in determining whether a country can or should be listed.