The young man testified at the International Criminal Court that he was abducted while on his way home from school and sent to brutal military training camps run by militia leader Thomas Lubanga. He was about 11 at the time, he told a three-judge panel.
Lubanga, founder and former leader of the Union of Congolese Patriots political movement and its armed wing, has pleaded innocent to charges of recruiting and using child soldiers in tribal conflicts in 2002-2003.
The witness spoke nearly two weeks after he started testifying and then retracted his evidence, apparently because he was scared of being in the same courtroom with Lubanga.
His identity has not been released and he was shielded from Lubanga in the courtroom by a curtain.
The witness gave a chillingly matter-of-fact account of a battle with fighters from the rival Lendu tribe near a missionary school.
"We went as far as the mission. At the mission we killed those who were there, also the priests," he said through an interpreter.
"We captured some of them, took them hostage," he added. "We cut their mouths off. We would destroy their faces. That's what the Lendu did too."
Prosecutors did not ask him to elaborate on the testimony.
The witness said he passed through Union of Congolese Patriots training camps where he learned how to shoot weapons and was regularly beaten by older soldiers.
"We were told that whoever lost his rifle would be beaten to death," he said.
He said he also was taught to smoke marijuana and drink beer. Prosecutors and rights groups say that armies that use child soldiers often drug them to calm the children's nerves and desensitize them to the horrors of battle.
The witness told judges he saw Lubanga land at an airstrip in a light plane that delivered uniforms and weapons.
Lubanga is the first suspect to face trial at the International Criminal Court, the world's first permanent war crimes tribunal. It was set up in 2002.
The court has only three other suspects in custody, all of them former Congolese warlords.
Judges are expected to announce within days whether they have agreed to a request by prosecutors to issue an arrest warrant for Sudanese President Omar al-Bashir on genocide and other charges that he masterminded atrocities in his country's Darfur region.