Four Mexican military vehicles, with more than 30 soldiers in uniform, crossed the U.S.-Mexico border without authorization on Tuesday.
American authorities are calling the incident an inadvertent incursion.
U.S. Customs and Border Protection spokesman Rick Pauza said no gunfire or injuries were reported when the 33 soldiers crossed the new Donna-Rio Bravo International Bridge over the Rio Grande, about 15 miles southeast of McAllen.
The soldiers were processed and returned to Mexico, he said.
"We have protocols in place to handle these types of incidents," he said in a statement. "... We continue to remain in constant communication with our Mexican counterparts, and we maintain a shared interest in keeping our mutual border secure."
Pauza declined to comment on what the soldiers were doing when they crossed the border, referring those inquiries to Mexican military officials.
Mexico has maintained a strong military presence in the border state of Tamaulipas, across the border from South Texas, after warfare broke out between the Gulf Cartel and the Zetas. The war erupted over the killing of a Zeta in the border city of Reynosa, across from McAllen, in January 2010.
Since then, Mexico's federal government has mounted a special operation to stop the violence with thousands of military and police reinforcements.
The Zetas are blamed for the massacre of 72 migrants nearly a year ago, then kidnapping bus passengers and burying them in mass graves.
This article is based on the Associated Press reporting.