US says Mexican helicopter crossed border

A Mexican military helicopter ventured about 100 metres into the United States and fired two shots, the US border security agency says in a claim quickly denied by Mexico.

The alleged incident took place on Thursday at 5.45am (1245 GMT), when "a Mexican law enforcement helicopter crossed ... north into Arizona", a US Customs and Border Protection statement said.

"Two shots were fired from the helicopter, but no injuries or damage to US property were reported."

The agency said the incident, which occurred about 13 kilometres southwest of the village of San Miguel, on the Tohono O'odham Indian Nation, "is currently under investigation".

The Mexican helicopter was on "a law enforcement operation near the border", it added.

Local media reported the helicopter was part of a counter-narcotics mission.

But criminal investigator Tomas Zeron de Lucio of the Mexican attorney-general's office denied the Mexican helicopter had crossed the border, and said the operation was "agreed and co-ordinated" with the US border patrol.

"The operation was on the border ... I do not think we crossed the border because we brought our navigation systems," he told reporters.

Zeron said the helicopter had not fired any shots but that instead, it was suspected drug runners who had shot at the aircraft.

He said that during the operation, conducted at a ranch suspected to be used as a border crossing for drugs and illegal migrants since 2006, Mexican security forces arrested four people and found 39 migrants, including 13 from outside Mexico.

They also disabled a radio antenna used by the drug traffickers to broadcast the presence of federal agents and confiscated packages of drugs and gun cartridges.

Earlier this year, a Mexican helicopter crossed the border near Sasabe, also in Arizona.