A top Democrat has expressed concern that Secretary of State Rex Tillerson has violated a law that's meant to stop foreign militaries from enlisting child soldiers.

In a letter to Tillerson last week, Sen. Ben Cardin, D-Maryland, noted that a report earlier this year said Tillerson had overruled recommendations from State Department officials and removed Iraq, Myanmar and Afghanistan from the annual list of countries that use or recruit child soldiers.

"I have since received troubling reports supporting those accounts, as well as a Dissent Channel message from career State Department officials arguing that the decision is inconsistent with U.S. law and will adversely affect efforts to protect children from being recruited and used as soldiers," wrote Cardin, the top Democrat on the Senate Foreign Relations Committee.

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Under the Child Soldiers Prevention Act of 2008, Cardin said that the State Department must publish a list of countries whose militaries recruit or use child soldiers. Countries that appear on the list are ineligible for certain military assistance, he said, including for financing, military education and training, direct commercial sales and foreign military and peacekeeping operations.

"Based on that dissent memo and the subsequent response from your staff, I am concerned about the precedent this action may set and the message it could send to countries that still have not ended child soldiering," Cardin said.

The State Department has not yet commented.

This comes after Reuters reported Tuesday about the confidential "dissent" memo in which a group of roughly a dozen State Department employees formally accused Tillerson of breaking the law. The report said that Tillerson decided in June to exclude Iraq, Myanmar, and Afghanistan from the list.

CBS News' Margaret Brennan contributed to this report.