U.S. officials say nearly three dozen Mexican soldiers in four military vehicles crossed the border into the United States in an incident being described as inadvertent.

U.S. Customs and Border Protection officials say the 33 soldiers crossed the new Donna-Rio Bravo International Bridge into southern Texas on Tuesday after being unable to turn around on the span. Officials say the soldiers were processed according to protocols, released and returned to Mexico without incident.

Mexico's Defense Department has said the soldiers were on patrol along the border when they crossed the bridge over the Rio Grande and into the U.S. They say that after the situation was clarified with U.S. immigration authorities, the troops went back to Mexico.

Mexican President Felipe Calderon has deployed nearly 50,000 troops in the crackdown against drug violence since he took office nearly five years ago. More than 37,000 people have been killed in the country's drug war since then.