Mexican President-elect Felipe Calderon said a US plan to erect a border fence to keep out illegal immigrants was "mistaken" and would complicate his White House visit next month.
US President George W Bush signed legislation on Thursday to build 700 miles (1,126 km) of fencing along the US frontier, despite massive street protests in recent months by opponents and condemnation by Mexico's government.
Immigration is seen as a threat to employment by many Americans.
Supporters of the fence say it is also needed to keep out drug smugglers and terrorists.
Mexico says the barrier, which would run along parts of California, Arizona, New Mexico and Texas, will be ineffective and cause more deaths along the border.
Mr Calderon said the fence makes US-Mexico relations more difficult.
He was due to visit with Mr Bush on November 9th, weeks before he is sworn in as Mexico's president.
"Without a doubt the relationship between Mexico and the United States is considerably more complicated because of this mistaken decision the Americans have made," Mr Calderon told reporters.
"It's going to be an enormously complicated visit," he said during a trip to Canada.
Every year, thousands of Mexicans risk their lives sneaking across the 2,000-mile (3,200-km) border, much of it desert, looking for work to escape poverty. The millions of illegal Mexican immigrants who work in the United States send billions of dollars home to their families every year.