President Barack Obama’s visit to Mexico this week is a signal of support for President Felipe Calderon and his efforts to confront violent drug trafficking gangs, White House officials said on Monday.

The officials, briefing reporters on Obama’s upcoming trip to Mexico and Trinidad, gave no indication the U.S. government was planning new initiatives on difficult issues like cross-border trucking or immigration before the visit.

"The stop in Mexico is meant to send a message … it’s a message of admiration for the courageous steps that President Calderon has undertaken," said Denis McDonough, director of strategic communications at the National Security Council.

"The president admires his work as it relates to confronting violence and impunity by criminal trafficking organizations," McDonough said.

Obama planned to stop in Mexico on Thursday before traveling to Trinidad and Tobago for the Fifth Summit of the Americas.

Asked about immigration, Dan Restrepo, a special assistant to the president on Latin America, said only that Obama has made it clear he believed changes were needed to ensure "humane treatment of immigrants in the United States, while enforcing the immigration laws."

Recent reports have indicated the president intends to begin a U.S. debate on immigration reform this year. The United States is home to 12 million illegal immigrants, mostly from Mexico and Central America. Efforts to reform the system, enabling some to find a path to legal residency, failed in Congress in 2006 and 2007.

In another dispute, Mexico slapped retaliatory tariffs on $2.4 billion worth of U.S. goods last month after Obama signed an omnibus spending bill that canceled a pilot program allowing Mexican long-haul truckers to operate in the United States, as required by the North American Free Trade Agreement.

Transportation Secretary Ray LaHood has sent the White House recommendations for ending the dispute, an administration aide said on Monday. But the officials declined to say whether Washington would announce its proposal before the Mexico visit.

"I don’t want to get in a position to either suggest that there will or there won’t be one," said McDonough. "What I am telling you is that we’re aggressively working it and when we get an agreement we’ll announce it."