Anthony Placido said that law enforcement in Mexico was "all too often part of the problem".

His comments came as Mexican forces stepped up patrols in eight cities along the US border in an operation against drug-related violence.

Correspondents say the authorities aim to show they have not lost control.

More than 500 people are said to have died so far this year in drug-related violence in northern Mexico.

For its part, Mexico has asked Washington to stop advanced weaponry falling into the hands of the drug gangs.

Mr Placido said the "single largest impediment to seriously impacting the drug trafficking problem in Mexico is corruption," in testimony prepared for a Congress panel hearing.

Flashpoint city

More than 1,000 Mexican troops and police are patrolling the cities on their side of the border.

The focus is Nuevo Laredo, where forces have surrounded police stations after being shot at as they entered the city.

More than 700 local officers have been suspended amid government accusations that they have aided drug-traffickers.

Nuevo Laredo is a major transit point for drugs entering the US and drug cartels have been fighting for control of the market.

Concerns about lawlessness were reignited last week after the city police chief was shot dead a day after he took up the post.