21 people have been killed in a gunfight between two of Mexico's most powerful drug cartels

21 people have been killed in a gunfight between two of Mexico's most powerful drug cartels near the border with the US, officials have said.

The incident occurred off a remote dirt track near the city of Nogales, in the northern state of Sonora, they said.

Nine people have been arrested, six of whom were wounded in the confrontation.

Correspondents say gangs are involved in smuggling drugs and people to the US, and have been battling for control of the area - a key trafficking route.

However, they rarely stage such large-scale shootouts, they add.

There was no indication about what specifically triggered Thursday's gunfight between the Sinaloa cartel, led by Joaquin "El Chapo" Guzman, and an offshoot, the Beltran Leyva cartel.