A Mexican law enforcement helicopter crossed the U.S.-Mexico border and fired into the Arizona Thursday morning, Customs and Border Protection said.

Mexican authorities dispute the claims.

At approximately 5:45 a.m. the helicopter flew about 100 yards into Arizona and two shots were fired, CBP said in a statement. The shooting occurred eight miles southwest of the Village of San Miguel on the Tohono O'odham Indian Nation.

While no injuries or damage were reported, a Border Patrol union spokesman said agents came under fire during the incident.

“One of their helicopters came across the border and fired at a marked Border Patrol unit,” said Shawn Moran, vice president of the National Border Patrol Council, who called the incident a breach of trust.

But Mexican authorities said Friday they did not believe the helicopter had crossed into Arizona and denied reports that any gunfire was directed at Border Patrol agents during an operation near the border town of Altar, an area known for smuggling.

Tomás Zerón de Lucio, director of the criminal investigation agency of the office of the Mexican Attorney General, said three people were detained during the operation, which targeted a ranch and a radio antenna used as a repeater by lookouts.

Mexican officials said smugglers opened fire as helicopters flown by federal police and military personnel swooped in. The only shots fired were by the smugglers, who had been using the ranch since 2006 as a way station for immigrant crossing and drug trafficking, Zerón de Lucio said.

Moran maintains the gunfire was directed at the agents. He said confrontations with Mexican authorities and Border Patrol have become more frequent over the years but said it was not a common occurrence.

In January, two armed Mexican soldiers engaged in a standoff with Border Patrol agents near Sasabe, the Los Angeles Times has reported.

In a letter written after the incident to Sen. Tom Coburn, ranking Republican on the Senate Homeland Security and Governmental Affairs Committee, CBP Commissioner R. Gil Kerlikowskecited 23 military incursions from Mexico in the Tucson and Yuma sectors of Arizona since 2010.

Customs and Border Protection said Thursday’s events were under investigation.