The Mexican government has condemned a tough new immigration law passed in the US state of Georgia on Monday.
The issue has polarised opinion in the US
Presidential spokesman Ruben Aguilar said the legislation discriminated against Mexicans and that diplomats would monitor how it was applied.
The law, which will take effect next year, prevents illegal immigrants from receiving many social services.
It will also require police and employers to report undocumented workers to the Immigration Service.
The signing of the legislation comes amid deadlock in the US Congress about how to deal with illegal immigration, as well as nationwide protests by worried Latino groups.
"The referred legislation incurs discriminatory acts against the Mexican population and those of Mexican origin," Mr Aguilar said.
"It is a partial measure that fails to resolve the complex phenomenon of immigration between Mexico and the United States in an integral manner."
Since becoming Mexico's president in 2000, Vicente Fox has been pressing for reforms that would allow more Mexicans to work legally in the US.
Georgia Governor Sonny Perdue, who signed the bill, said it was intended to ensure that everyone who lives in Georgia abides by the laws there.
"It is our responsibility to ensure that our famous Georgia hospitality is not abused, that our taxpayers are not taken advantage of and that our citizens are protected," Mr Perdue.
"I want to make this clear: we are not, Georgia's government is not, and this bill is not anti-immigrant," Gov Perdue said.
The law, which will take effect in July 2007, will also impose prison terms for human trafficking and limit the services commercial companies can provide to illegal immigrants.
BBC correspondents say the move has come at a difficult time in the US as there have been country-wide protests over a federal immigration law currently being debated in the Senate.
"It is a punitive bill," said the president of the Georgia Hispanic Chamber of Commerce, Sara Gonzalez.
"This is a very complicated issue, and I don't see any good coming out of this."
There are an estimated 11 million undocumented workers in the US.