A published report says the U.S. government is aiding elite units of Mexican marines to pursue drug cartel bosses in aggressive "capture or kill" missions.
The Washington Post cites unidentified diplomats and law enforcement officials as saying the training and information-sharing is at a stronger level than previously known.
The newspaper reports Mexican officials have denied the U.S. military is training Mexican marines, while the U.S. Defense Department has declined to discuss the training.
However, U.S. diplomatic cables published by WikiLeaks have described the government's training effort. The cables have been posted on Mexican web sites and reported in major U.S. newspapers.
The Post says the leaked diplomatic cables reveal the U.S. is conducting urban-combat and counterinsurgency instruction in Mexico.
The newspaper reports that Mexican security analysts, U.S. officials and former military commanders have said the marines are "recipients of an unequaled level of cooperation" with American military and anti-narcotic agencies.
The cables released by WikiLeaks reveal how the U.S., through its embassy in Mexico City and its consulates along the border, delivers intelligence from the U.S. Drug Enforcement Administration to Mexican marines about the location of drug bosses.
The information has allowed the marines to capture and often kill drug kingpins in what are sometimes spectacular urban shootouts.
Nearly 30,000 people have been killed in Mexico's drug war since President Felipe Calderon took office in late 2006 and began cracking down on the cartels.