AFP | WASHINGTON — US National Guard troops being sent to the Mexican border will be used to stem the flow of guns and drugs across the frontier and not to enforce US immigration laws, the State Department said Wednesday.

The clarification came after the Mexican government urged Washington not to use the additional troops to go after illegal immigrants.

President Barack Obama on Tuesday authorized the deployment of up to 1,200 additional troops to border areas but State Department spokesman Philip Crowley told reporters, "It's not about immigration."

He said the move was "fully consistent with our efforts to do our part to stem, you know, violence, to interdict the flow of dangerous people and dangerous goods -- drugs, guns, people."

He said the extra troops would be used to free up civilians engaged in support functions so that law enforcement personnel can be increased along the 2,000-mile-long (3,200 kilometer) border.

Nearly 13 million Mexicans live in the United States, more than half of them illegally.

"We have explained the president's announcement to the government of Mexico, and they fully understand the rationale behind it," Crowley said.

Obama's announcement came less than a week after a state visit to Washington by Mexican President Felipe Calderon, who asked for greater US backing for a bloody three-year-old war on drug cartels.

Drug violence has claimed the lives of nearly 23,000 people over the past three years.