McCain, the presumptive Republican candidate for president, labeled the so-called Merida initiative approved by Congress last week an "important sign of progress and cooperation to fight the drug cartels and the flow of drugs into the United States."

The aid measure will provide $400 million to Mexico in police training and equipment next year.

The Arizona senator spoke after touring the high-tech command center of a gleaming new federal police facility.

He met earlier with Mexican President Felipe Calderon and said the two had discussed immigration and trade. McCain has sought to highlight differences with his Democratic opponent, Barack Obama, over the trade issue.

McCain is a strong backer of the North American Free Trade Agreement, which covers the United States, Mexico and Canada, and he supports a free-trade pact with Colombia.

Obama opposes the Colombia agreement without labor and human-rights improvements in that nation, and favors renegotiating NAFTA to ensure better labor and environmental safeguards.

McCain spent 24 hours in Mexico after a two-day visit to Colombia, where he praised that nation's drug-interdiction efforts.

In his Mexico stop, McCain reiterated his support for allowing more immigrant workers to enter the United States on a temporary basis. But he said broad immigration reform should come only after the U.S. government has tightened the border adequately, including by building fences.

Migration is a big issue in Mexico, the main source of undocumented immigrant labor in the United States. Mexican migrants in the United States sent home $24 billion in remittances last year. "We must have comprehensive immigration reform, but the American people want our borders secured first," McCain said.

McCain's visit to Mexico, seen by some commentators here largely as a play for Hispanic votes in the United States, came as the Calderon government has pursued a crackdown on drug trafficking. That effort, and scattered turf wars among traffickers on the U.S. border and elsewhere, have fed a spiraling death count put this year at more than 1,750.

Calderon says the government campaign, which has sent more than 40,000 troops and 5,000 federal police agents into the streets, is working. He and other officials say the arrests of several key trafficking figures and drug seizures have delivered a blow to the drug trade.

Trafficking gangs have taken on Mexican law enforcement, and one another, with increasing brutality. Some ranking federal police officials are among an estimated 450 officers and soldiers slain since Calderon announced his anticrime drive in December 2006.

The U.S. aid package is intended to strengthen Mexican security forces, who in many cases are outgunned by drug cartels. Improved training is aimed at creating more professional police forces, starting at the federal level.

McCain told the federal officers that their efforts against drug traffickers mattered to both nations. "We are in a common struggle," he said, "with a common enemy."