The Mexican navy denied Monday that the shots which killed civilians caught up in a clash between security forces and gunmen were fired from one of its helicopters.

The helicopter was deployed in the wee hours of Sunday after gunmen ambushed members of the security forces near Nuevo Laredo, Tamaulipas state, just across the border from Laredo, Texas.

Media outlets in the area reported that the chopper was the source of the gunfire that struck the civilians and that marines blocked the path of an ambulance arriving to attend to a husband and wife and their two children.

"The bullet impacts that the civilian personnel received were due to crossfire at ground level, and not from the air," the navy said in a statement.

The bullets that killed the family were a different caliber from the ammunition used for the gun mounted on the chopper, the navy said.

"Naval personnel provided the medical attention necessary in the area of the incident," according to the statement, which said that the purported adult male victim was actually alive and being treated at a Nuevo Laredo hospital.

Five combatants - one service member and four gunmen - died in the clash and 12 other marines were wounded.

Tamaulipas, one of Mexico's most dangerous states, has been rocked for years by battles among rival drug cartels and between the criminals and the security forces.