President Barack Obama and Mexican President Felipe Calderon are meeting amid heightened tensions over violence at their shared border, though no major breakthroughs are expected on what's quickly becoming a defining issue in the relationship between the North American allies.

Administration officials said the White House has been working closely with Calderon's government on how to protect U.S. personnel working in Mexico, but they wouldn't say whether Obama would press the Mexican leader to allow U.S. agents to be armed.

U.S. and Mexican officials have emphasized that Calderon's visit was planned before Zapata's killing. The leaders are expected discuss a wide range of topics, including economic cooperation and immigration, during a private meeting before taking questions from reporters.

The contentious debate over immigration dominated Calderon's visit to the White House in May, shortly after Arizona passed a law that makes it a state crime to be in the U.S. illegally and requires police to question people about their immigration status if there's reason to suspect they're illegal. Mexico's government strongly opposes the law, and the Mexican Senate this week urged Calderon to again vehemently make their opposition known to Obama.

Obama also opposes the Arizona law; the White House said that comprehensive immigration reform remains high on Obama's agenda and that the president would update Calderon on the state of the immigration debate in the U.S.

Negotiations also are continuing between the two countries over opening U.S. highways to Mexican trucks, as was agreed to in the 1994 North American Free Trade Agreement. U.S. truck drivers oppose allowing Mexican motor carriers access to the U.S., saying the Mexicans have an economic advantage because they don't have to meet as stringent safety and environmental standards.