Mexico City, Jul 7 (IANS/EFE): At least six gunmen were killed when they attacked a navy helicopter in the northeastern Mexican state of Tamaulipas, the navy secretariat said on Monday.

The chopper was carrying marines on a surveillance mission on Sunday about 70 kms south of the border city of Nuevo Laredo when it came across several vehicles carrying gunmen, who opened fire on the aircraft, the secretariat said in a statement.

The helicopter was hit six times, but the occupants were not injured.

The marines were searching the stretch between Sabinas Hidalgo, a city in Nuevo Leon state, and Nuevo Ciudad Guerrero, a city in Tamaulipas, for criminals who naval intelligence determined operate in the area.

The gunmen opened fire on the helicopter as soon as they were spotted by the crew.

Navy personnel fired several "warning shots to stop the assailants, who ignored them and continued firing, so the attack was repelled, leaving six people dead among the assailants," the secretariat said.

Marines seized four vehicles and six firearms following the incident.

Tamaulipas has been a battleground between the Gulf and Los Zetas drug cartels for years, and it is regularly among the states with the highest number of homicides.

President Enrique Peña Nieto sent additional federal police and military personnel to Tamaulipas in May 2014 and ordered a thorough vetting of the state and municipal police forces to root out corrupt officers.