A 14-year-old boy suspected of acting as a killer for a drug cartel has been arrested by the Mexican army.

The boy said he had been working for the cartel since he was 11.

The alleged young assassin, nicknamed "El Ponchis" - the Cloak, was captured at the airport near Cuernavaca with his 16-year-old sister as they tried to catch a flight to Tijuana and flee the country, said an army official.

The sister told reporters that they planned to cross the border to San Diego, California, where their mother lives.

The two were brought to the office of the Mexican Attorney General in Cuernavaca, where the boy told reporters that he had taken part in at least four decapitations.

His sister was accused of getting rid of the bodies by dumping them on streets and motorways.

"I participated in four executions, but I did it drugged and under threat that if I didn't, they would kill me," said the teen, who appeared calm and showed no remorse.

The attorney general for Morelos state said the two would be turned over to state authorities, who handle crimes committed by minors in Mexico.

The two were suspected of helping the South Pacific Cartel headed by Hector Beltran Leyva, brother of Arturo Beltran Leyva, a top drug lord who was killed by Mexican marines in Cuernavaca a year ago.