The US president is planning to send up to 1,200 National Guard troops to the US-Mexican border to curb illegal immigration and drug trafficking, officials say.
Barack Obama's move, for which $500m is being sought, follows demands from both Republicans and Democrats for more federal resources along the frontier.
The announcement comes as the president seeks Republican support for a sweeping overhaul of US immigration laws, and seeks to rally opposition to a tough new immigration law in Arizona that has caused tension in US relations with Mexico.
The troops will provide intelligence, surveillance and reconnaissance support and intelligence analysis, according to officials.
They will also provide immediate support to counternarcotics enforcement and training capacity until the Customs and Border Patrol agency can recruit and train more border officers and agents, the official said.
The $500m will be used to enhance technology at the border and share information and support between law enforcement agencies to crack down on illegal trafficking in people, drugs, weapons and money.
"The Mexican government considers that the decision ... should translate into the channeling of additional resources to make efforts more effective to combat the trafficking of illegal arms"
Mexican foreign ministry statement
Arizona senators - John McCain and Jon Kyl - and state governor Jan Brewer, all Republicans - have asked Obama for more federal support to secure the border.
McCain and Kyl introduced a Senate amendment to force Obama to deploy 6,000 National Guard troops to the border.
"Though this initial deployment is an important first step, the president is not sending enough troops," the senators said late on Tuesday in a statement.
The administration rejected the request in a letter to Carl Levin, the chairman of the Senate armed services committee.
"It represents an unwarranted interference with the commander-in-chief's responsibilities," wrote national security adviser James Jones and counterterrorism chief John Brennan.
Illegal immigration across the border with Mexico has been in intense focus since Arizona passed a law that requires state and local police to investigate the immigration status of people they reasonably suspect are in the country illegally.
The law also seeks to drive 460,000 illegal immigrants out of Arizona, which straddles one of the principal corridors for human and drug smugglers heading up from Mexico.
It was a central issue last week during a state visit to Washington by Felipe Calderon, the Mexican president, who said the law discriminated against foreign-born workers.
Mexico urged the US on Tuesday to use the troops planned for deployment to pursue criminals, rather than migrants.
Mexican officials said they respected Obama's right to make the decision, but thought Washington should address problems originating on its side of the border.
"The Mexican government considers that the decision ... should translate into the channeling of additional resources to make efforts more effective to combat the trafficking of illegal arms and cash to Mexico," the Mexican foreign ministry said in a statement.
Mexico also urged "shared responsibility" on the fight against drug gangs along the border.
There are currently 344 US National Guard troops working along the border.