Washington/Mexico City - US Attorney General Eric Holder, in Mexico Thursday to meet his Mexican counterpart Medina Mora, vowed to stand "shoulder-to-shoulder" with Mexico in the fight against the drug cartels.

"The United States shares responsibility for this problem and we will take responsibility by joining our Mexican counterparts in every step of this fight," Holder said.

In 2008, more than 6,300 murders in Mexico were blamed on drug cartel conflicts, including the deaths of more than 500 police and military units.

Mexican President Felipe Calderon has deployed 45,000 army troops to combat the scourge, and cities like Juarez, near the US-Mexico border, are under martial law.

Holder said that the fight would be won "thanks in large part to the courage of my Mexican colleagues ... who are on the front lines every day."

Holder's high-profile meeting scheduled for Friday with Mexican President Felipe Calderon underlined the efforts of the Barack Obama administration to get on good terms with the US' southern neighbour after years of soured relations.

US Secretary of State Hillary Clinton, on a visit to Mexico last week, made the first ever public admission by the US that it must accept part of the blame, for the drug-related violence that is ravaging Mexico, because of the "insatiable demand" for illegal drugs in the US.

Also last week, Washington launched a major new effort to crack down on the cartels by beefing up drug and weapons agents along the border and sending help into Mexico in the form of trainers for the justice system and equipment such as helicopters.

Mexican cartels are major suppliers of methamphetamine, cocaine and heroin to the US.

Holder and US Homeland Security Secretary Janet Napolitano met with Mora and other Mexican officials in Cuernavaca, Mexico, about 85 kilometres south of Mexico City.

Holder told Bloomberg financial news service that the US needs to determine the source of the guns going into Mexico.

"That has got to be, I think, a prime responsibility for our country," Holder said.

More than 90 per cent of guns used in violent crimes in Mexico are brought in illegally from the United States, the US Bureau of Alcohol, Tobacco, Firearms and Explosives says.

The governments of Mexico, Colombia and other countries beset by the drug trade have also pushed the US to cut down on the demand for drugs in the US through treatment and education programmes.

Napolitano said Thursday's meetings were "focused on the continued role the United States will play in helping Mexico to suppress cartel violence."

Obama is to visit Mexico on April 16-17 on his way to the Americas summit in Trinidad and Tobago.