Mrs. Clinton traveled to Mexico City and the central town of Guanajuato for meetings with Mexican President Felipe Calderón and Foreign Minister Patricia Espinoza. The State Department said the meetings focused on a range of issues, from collaboration against drug traffickers to immigration, economic strategy and the environment.

But the battle against organized crime was expected to be the main topic of discussion.

Under the Obama administration, Mexico's importance in U.S. foreign relations has grown. More than 34,000 people have died in drug-related violence in Mexico since 2006, many in cities that border the U.S. Relations between the two countries have been strained by a raft of proposed U.S. immigration laws that aim to identify, prosecute and deport illegal immigrants. And officials are trying to negotiate a deal to end a costly spat that has restricted Mexican truck drivers in the U.S.

Andrew Selee, an expert on U.S.-Mexico relations at the Woodrow Wilson Center, said these kinds of high-level encounters are relatively new, a product of the two countries' closer cooperation than in previous years. "There are lots of moving pieces now, from intelligence-sharing to borders and ports," he said.

The WikiLeaks cables show a close working relationship between the governments on trying to combat Mexico's powerful organized crime. They also shed light on Washington's frustration with Mexico's strategy of using the military as the main tool against cartels instead of creating robust police forces.

The cables, from El País, detailed a February 2010 meeting in which Secretary of Homeland Security Janet Napolitano pressed Mr. Calderón on progress in the border town of Ciudad Juárez.

In a response to requests by Mr. Calderón for more assistance, Ms. Napolitano said the U.S. government could only "help identify the right organized-crime targets, but that Mexico must move beyond military deployments and establish a police capacity in Ciudad Juárez." The city is Mexico's most deadly, the site of roughly 3,100 deaths last year.

In the discussion, Mr. Calderón admitted that the strategy of sending thousands of soldiers into the city in March 2009—a centerpiece of the president's national strategy—hadn't worked as planned because "crime exploded as kids fought each other on the street to control drug trade." The result led the president to remove patrols from the center of the city. He also cast doubt on future deployments of the military in other locations "in fear that they will be corrupted," according to the cables.

The cables also mention a troubled situation at Mexico's southern border with Guatemala, another key smuggling point. Mr. Calderón and Ms. Napolitano agreed that region's jungles made security difficult. Instead, Mexico has focused on setting up "choke points" to try to catch traffickers after they have already arrived in the country, the cables said.

The region is home to a group known as Los Zetas, who authorities said killed a group of 72 immigrants, including many Central Americans, headed to the U.S. last year.