TUCSON, Ariz. (AP) - Mexican law enforcement on Thursday crossed into Arizona by helicopter and fired two shots at U.S. border agents, a border patrol union leader says.

A Mexican law enforcement chopper crossed about 100 yards north into the Arizona desert, the U.S. Border Patrol said in a statement. The helicopter then fired two shots on the Tohono O'Odham Indian Nation, which sits on the border. Border patrol union leaders say the Mexicans fired at agents but that none of them were hurt.

However, Mexican authorities have denied shooting at agents and say they were under attack during a mission to find smugglers on the border.

Toms Zern, the director of the Mexican attorney general's office investigative office, said that Mexican military and federal police who were conducting an operation on a ranch in Altar, Sonora, were shot at by criminals. Mexican authorities never fired any weapons and in fact never crossed into the U.S. side of the border, he said.

Art del Cueto, president of the local border patrol union, said four agents were shot at when they were in a marked patrol vehicle.

"They could say they didn't fire at the agents intentionally. But for them to say that they were no shots fired within the United States, toward the United States Border Patrol, is a lie. They got in contact with our managers and apologized for the incident," del Cueto said.

The Mexican helicopter was 15 yards from the border agents when they were came under fire, Del Cueto said.