Mexican authorities have arrested a 14-year-old US citizen suspected of being a drug cartel hitman, in a week that has seen the arrest of a number of major cartel leaders along with the discovery of a mass grave in the country’s northern regions.

Edgar Jimenez, nicknamed El Ponchis (techno/house music), became the focus of international media attention when videos of him slitting a victim's throat, beating another with a stick and posing by corpses and weapons appeared online a month ago.

"I participated in four executions, but I was drugged and under threat that if I didn't, they would kill me," Mr Jimenez told reporters as he was paraded in front of them.

He was arrested as he prepared to board a flight to Tijuana, northern Mexico, from the city of Cuernavaca, just south of Mexico City. According to authorities, the teenager was preparing to meet his stepmother in San Diego, his birthplace.

The boy is believed to be a member of the South Pacific Cartel, which formed in April and is fighting La Familia for control of southwest Mexico. The group is allied with Los Zetas, a notoriously violent group of special forces deserters.

He was captured alongside two of his teenage sisters. One of them is believed to be the girlfriend of a leader of the Beltrán-Leyva Cartel whose former leader Arturo Beltrán Leyva was killed a year ago by Mexican authorities in Cuernavaca. His death led to a surge in violence in the wealthy region.

Mr Jimenez claims to have worked for a drug cartel since he was 11 and is said to have participated in at least four decapitations.

Last week was a coup for Mexican authorities with the capture of alleged leaders of Los Zetas, La Familia and Los Aztecas—footsoldiers for the Juárez cartel.

However, successes were overshadowed by the discovery of a mass grave in northern Mexico. In it were 20 bodies that included two US citizens.

Meanwhile, cables released by whistleblowing website Wikileaks show US frustration with Mexican authorities, in contrast to the US' public statements of support for the government of President Felipe Calderón.

"Mexican security institutions are often locked in a zero-sum competition in which one agency's success is viewed as another's failure, information is closely guarded, and joint operations are all but unheard of," said a January cable, signed by John Feeley, Deputy Chief of Mission at the US Embassy in Mexico City.

"Official corruption is widespread," he continued. "Prosecution rates for organised crime-related offences are dismal."

Another cable tells of Mexican General Guillermo Galvan Galvan’s suggestion to bring martial law in certain areas of the country in order to combat drug-related crime.

US ambassador to Mexico Carlos Pascual said that the cables should be viewed as “impressionistic snapshots of a moment in time”. Speaking to the LA Times, Mr Pascual continued: “But like some snapshots, they can be out of focus or unflattering.”

The Mexican government condemned the leaks. “The set of leaked documents and their contents are incomplete and inaccurate,” read a statement. “The leaked documents respond to specific events or times with no narrative context, which prevents proper setting and weigh their relative importance.”

The number of deaths in Mexico's drug war is on par with US deaths in Vietnam, according to Mexican Senate Chairman Manlio Fabio Beltrones.

Over 30,000 people have died in Mexico since President Felipe Calderón took office in 2006, and the violence has no signs of abating.

“We have seen 30,000 people die in the past four years and we are very close to seeing, at the end of this six year term, an equal number of dead as in the Vietnam War,” Beltrones told CNN. “Who really thinks that with the next 30,000 we will solve the problems of crime and violence?”

The US military saw 58,000 deaths between 1955 and 1975 in Vietnam.

US Secretary of Homeland Security Jane Napolitano failed to mention the drug wars in a speech given in Mexico City last week. She focused on a frequent flyer programme to be enacted between the two countries, allowing easier passage through immigration and customs.