The lack of a comprehensive security strategy for the U.S.-Mexico border hampers the ability of the Department of Defense to make the best use of guardsmen assigned to help the Border Patrol, a federal report says.

The new report by the Government Accountability Office also says defense officials are "concerned about 'mission creep' " because border security is not a core mission of the National Guard.

The GAO report comes after last week's announcement by Homeland Security officials that they would extend the current National Guard border mission for another three months. It is the second mission extension for the guardsmen since the current border assignment began in July 2010.

The current border mission is the second time in the last five years that guardsmen have been sent to the border. In Operation Jump Start of 2006-08, as many as 6,000 soldiers were stationed along the U.S.-Mexico border. There are now 1,140 guardsmen along the border, with about 500 in Arizona.

The two missions have cost $1.35 billion, the GAO reported. The latest extension is expected to cost $35 million.

In a Senate hearing Tuesday, John McCain, R-Ariz., asked Homeland Security Secretary Janet Napolitano about the Defense Department concerns raised in the GAO report.