Department of Homeland Security Assistant Secretary Alan Bersin, who visited Arizona's busiest commercial port here on Wednesday, said those concerns have triggered a series of border security initiatives and brought about closer cooperation with Mexican federal authorities.

"We take the threat of spillover violence very seriously," Bersin said. "We're prepared to deal with it in the event it occurs. There are contingency plans to respond. But we have not yet seen that violence spill over into the United States."

He noted that since 2006, there have been 11,000 deaths in Mexico related to violence between competing drug cartels and the government's challenging the cartels.

"That's not to say that we don't have plenty of violence in the United States that's linked to the activities of Mexican drug and alien-smuggling organizations based in Mexico. We do."

Bersin said DHS Secretary Janet Napolitano, formerly Arizona's governor, is constantly monitoring the violence for any signs of spillover.

He added that a targeting of U.S. law enforcement officials _ such as has occurred with frequency and deadly effect against their Mexican counterparts _ would indicate such a spillover.

Bersin said the border safety and security strategy involves technology, agent deployment, intelligence and infrastructure in a series of contingency plans in the event of a major incident. He declined to elaborate.

Bersin said Napolitano focused "laser-like" on the problem, instituting a series of responses that began with the Southwest Border Initiative in late March to gear up responsive efforts should a spillover occur.

Any decision on whether National Guard troops would be used in the event of heightened cross-border violence will be made by President Barack Obama based on recommendations of Napolitano and Defense Secretary Robert Gates, he said.

The two secretaries have had an ongoing dialogue on the issue since earlier this year, he noted. Bersin pointed out that the National Guard has had a presence supporting law enforcement along the border for more than two decades.

On another border enforcement issue, the U.S. Senate last week approved a Republican-sponsored amendment to a DHS budget measure that would require double fencing along nearly 700 miles of border pedestrian fencing and vehicle barriers. The measure focused on double fencing rather than additional high-tech so-called virtual fencing.

"The fence, taken in concert with deployment of agents and technology, provide a much more effective operational control of much more of the border than we've ever seen before," Bersin said.

"But all of the use of the fencing has been done on the advice of the Border Patrol. ... Fences and other infrastructure have an important role to play but only in concert with these other elements."

He said Napolitano wants every fence and other infrastructure put into place "to be recommended by the professionals, in concert with these other elements."

The border chief said U.S. Customs and Border Protection officers making southbound checks of both Americans and Mexicans at ports along the Mexican border have seized 654 weapons, largely handguns, and have seized more than $28 million in bulk cash since mid-April.

Mexico, meanwhile, has committed $1.4 billion to upgrading its customs operations, all part of a recognition that both countries share responsibility for drugs going north and guns and cash going south, Bersin said.

For the first time, he said, Mexico is transforming its customs duties from exclusively generating revenue and facilitating trade to assuming an enforcement posture.

Among other things, Mexico has installed sophisticated license plate reader devices to check all incoming traffic at some border ports adjacent to Texas, and they should be in place from Matamoros to Tijuana by the end of the year, he said.