US Secretary of State Hillary Clinton, on a visit to Mexico, has reiterated her support for President Felipe Calderon's fight against drugs gangs.
Mrs Clinton said there was "no alternative" to confronting the cartels, despite rising violence that left more than 15,000 dead last year.
She said the US recognised the need to stop the flow of money and guns from the US to the cartels.
Mrs Clinton later met Mr Calderon and discussed security and economic issues.
Earlier she met her Mexican counterpart, Patricia Espinoza, in the city of Guanajuato.
Mrs Clinton said Mexico's policy of fighting the drugs gangs was "absolutely necessary," despite the high cost in bloodshed.
"It is messy. It causes lots of terrible things to be on the news," she said. "The drug traffickers are not going to give up without a fight".
Mrs Clinton praised the Mexican government's efforts to reform the judicial and prison systems, and promised to maintain US aid for the fight against organised crime.
Under a security cooperation programme called the Merida Initiative the US is spending around $1.7bn (£1bn) on helping Mexico and Central America tackle drug-trafficking.
The BBC's Julian Miglierini in Mexico City says Mrs Clinton's visit is being viewed, in part, as an exercise in damage limitation following revelations by the whistleblowing website, Wikileaks.
Secret US diplomatic cables released by Wikileaks in recent weeks suggest US officials are privately sceptical about the ability of some Mexican security agencies to tackle the cartels.
One leaked cable, apparently sent in 2009, described Mexico's intelligence apparatus as "fractured, ad hoc and reliant on US support".
Since President Calderon launched a military offensive against the drug gangs in December 2006, violence has increased, especially in northern border areas, with more than 34,000 people killed.
Last year was the bloodiest so far, with 15,273 drug-related killings, according to Mexican government figures.
The Mexican authorities argue that the rising violence shows that the gangs are being weakened and turning increasingly on each other, but critics argue the use of troops has only served to provoke increasingly gruesome murders.
Also on Mrs Clinton's agenda in Mexico were economic issues, including the access of Mexican truck drivers to the US market, immigration and border control, and climate change.
After Canada and China, Mexico is the US's biggest trading partner, and is the second biggest market for US goods.