At least 21 people have been killed in a gunfight between two of Mexico's most powerful drug cartels near the border with the United States

The violence occurred off a remote dirt track near the city of Nogales, in the northern state of Sonora. Nine people have been arrested, six of whom were wounded in the confrontation. Gangs are involved in smuggling drugs and people to the United States, and have been battling for control of the area - a key trafficking route. There was no indication about what specifically triggered the gunfight between the Sinaloa cartel, led by Joaquin "El Chapo" Guzman, and an offshoot, the Beltran Leyva cartel. All of the victims were believed to be members of the gangs. The fact that the violence happened only 12 miles from Arizona will do little to allay fears that Mexico's unrest is getting increasingly close. Recently, in the northern border city of Ciudad Juarez, gunmen killed Sandra Salas Garcia, an assistant attorney-general of Chihuahua state, and one of her bodyguards. Meanwhile, a severed head was reportedly left outside the home of the leading candidate for the mayor of Ciudad Juarez, Hector Murgia. For months, candidates have complained of intimidation by the gangs and have been forced to limit their campaigning in high-risk areas. Rodolfo Torre Cantu, who was running for governor in the north-eastern state of Tamaulipas, was shot dead along with four of his supporters near Ciudad Victoria. Drug-related violence has surged and in 2010 is on track to be the deadliest in half a decade, with some 5,000 killings so far - more than the totals in 2007 and 2008. Nearly 23,000 people are thought to have been killed since December 2006.