Mexico has become a major source for heroin in the US in recent years, according to the White House national drug threat assessment report.

It says that while heroin use is stable, or decreasing, in the US, the source of the drug has shifted from Colombia - where production and purity are declining - to Mexico.

Increasing violence

Heroin production in Mexico rose from 17 pure metric tonnes in 2007 to 38 tonnes in 2008, with the increase translating to lower heroin prices and more heroin-related overdoses and more overdose deaths, according to US government estimates.

Mexico and the US are working together to counter a handful of increasingly violent drug cartels that supply most of the illicit drugs sold in the US.

Medina's arrest came the day after senior US cabinet officials, led by Hillary Clinton, the US secretary of state, visited Mexico to highlight their shared responsibility for the country's drug-related violence.

The battle against drug trafficking has become increasingly violent.

On Thursday masked drug "hit-men" were reported to have terrorised residents and set fire to homes in a cluster of towns near Ciudad Juarez, in a bid to secure trafficking routes into the the US.

Ciudad Juarez lies across the border from El Paso, Texas.

Nearly 17,900 people have died in drug-related violence since Felipe Calderon, Mexico's president, launched an assault on cartels after taking office in December 2006.