WASHINGTON — In 2009, the government of Chad conscripted refugee children for unlawful use as guards and combatants in its desert battles against rebel forces; the army of the Democratic Republic of Congo forced children to carry ammunition and supplies through the jungle, and some died under their weight; hundreds of boys and girls were forced into the army of southern Sudan, despite a commitment to release them; and in Yemen, children as young as 14 make up perhaps half the ranks of both the government’s forces and the rebels opposing them.

Despite those findings, in an annual State Department report on human trafficking, the Obama administration is allowing American military aid to continue to the four countries, issuing a waiver this week of a 2008 law, the Child Soldiers Prevention Act.

In a memorandum to Secretary of State Hillary Rodham Clinton on Monday, President Obama said he had determined that the waiver was in “the national interest.”

The memo offered no elaboration. But administration spokesmen said that the law, signed by President George W. Bush but effective only as of this year, would have penalized countries providing crucial cooperation with the United States, including in the fight against Al Qaeda militants. In some cases, they said, it was easier to press countries to stop using young soldiers if the United States remained closely engaged with them.