severed head was reportedly left on Thursday outside the home of the leading candidate for the mayor of Ciudad Juarez, Hector Murgia.

severed head was reportedly left on Thursday outside the home of the leading candidate for the mayor of Ciudad Juarez, Hector Murgia. Fourteen states will hold elections on Sunday.

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 on Monday.Earlier this month, President Felipe Calderon deployed thousands more troops to the worst-affected areas along the US border as part of his "war on drugs".

In a televised address, he appealed to the nation to help combat the escalation of violence which has shaken the country.

"This is a battle that is worth fighting because our future is at stake," he said. "It's a battle that, with all Mexicans united, we will win."

Last month, Mr Calderon asked a joint session of the US Congress for support, saying it was crucial that the flow of weapons from the US to Mexico was curbed.

Despite his appeals, drug-related violence has surged and this year 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 President Calderon launched his crackdown on the cartels in December 2006.