Some 5,000 Mexican troops have been deployed in Ciudad Juarez
The US has said it is considering using the National Guard as a last resort to counter the threat of drug violence in Mexico spilling over the border.
The plan was outlined by Roger Rufe of the Department of Homeland Security in a House of Representatives hearing.
US state governors with borders joining Mexico have expressed growing concern over the impact of Mexican cartels which have links to US gangs.
There were more than 6,000 drug-related killings in Mexico in 2008.
Many of the battles over turf and smuggling routes - sometimes involving decapitations - are fought using US weapons smuggled south to Mexico.
Officials say Mexican drug gangs are active in 230 American cities.
Phoenix, Atlanta and Birmingham are among the hardest hit, with a big increase in kidnappings and murders.
Arizona Governor Jan Brewer has asked the federal government to send 250 National Guard reserve troops to help 150 already there supporting local law enforcement efforts against drug trafficking.
Texas Governor Rick Perry has asked for 1,000 National Guard troops.
Mr Rufe told a House homeland security subcommittee that the deployment of US military personnel and equipment would be a last resort if other agencies became overwhelmed.
"We would take all resources short of DoD (Department of Defence) and National Guard troops before we reach that tipping point," he said.
"We very much do not want to militarise our border."
But, he said, his department was engaged in planning with the National Guard and defence department "to make sure they're ready when the time comes".
On the Mexican side of the border, President Felipe Caldeon has sent thousands of extra troops into the city of Ciudad Juarez, just across the Rio Grande from the Texan city of El Paso.
Mr Rufe's comments echoed those made by US President Barack Obama a day earlier, when he said he did not have a "tipping point" in mind for sending in the military.
"We've got a big border with Mexico," he said on Wednesday. "I'm not interested in militarising the border."
Last year, a US justice department report described Mexican drug traffickers as America's biggest organised crime threat, a view reiterated by Mr Rufe.