US officials say two American soldiers in a remote area of Texas were confronted by Mexican soldiers who thought they had crossed into Mexico.
The Mexican troops reportedly took the weapon from the American soldier who was armed.
US defense officials say the Americans were in a Customs and Border Protection vehicle in a remote area southeast of El Paso, Texas, when the incident occurred.
The incident happened on April 13 and was first reported by Newsweek.
A Northern Command statement to The Associated Press on Tuesday said the Mexican military members believed they were on Mexican territory at the time they confronted the Americans.
The US troops are at the border as part of a Trump administration effort to reduce illegal crossings.