U.S. State Department spokeswoman Marie Harf said during her regular press briefing Thursday the government didn't issue a waiver to Rwanda from the Child Soldiers Prevention Act of 2008.
"The Rwandan government support for the M23, a rebel group which continues to actively recruit and abduct children, threatens peace and stability in the eastern DRC [is] the reason for not having a waiver," she said.
Rwanda has said allegations of supporting M23 were misguided, adding many of the problems in Democratic Republic of Congo require political, not military, solutions.
The U.N. Stabilization Mission in DRC said last month at least 80,000 people have been forced to flee because of a new outbreak of violence in the country's eastern provinces.
Fighting between M23 and government forces, who are supported by U.N. peacekeepers, has displaced more than 100,000 people in a country where a humanitarian crisis has impacted the lives of an estimated 2.6 million civilians.