WASHINGTON, --President Donald J. Trump has directed the Defense and Homeland Security departments to work with state governors to deploy National Guard troops to the southern U.S. border, a top official said today.Speaking to reporters at the White House, Homeland Security Secretary Kirstjen M. Nielsen said the troops would augment U.S. Customs and Border Protection and provide support where needed to secure the border with Mexico.Trump is issuing a proclamation today on the matter, she said.In a statement, chief Pentagon spokesperson Dana W. White highlighted DoD support for border security efforts."We are working with the Department of Homeland Security to enhance the support we provide to the DHS border security mission," White said. "We look forward to continuing our partnership with DHS to ensure the defense of our nation and security of the American people."'UNACCEPTABLE LEVELS' OF ILLEGAL ACTIVITYNielsen said the deployment is aimed at filling the gaps in border security and stopping illegal activities that pose a threat to the country."We continue to see unacceptable levels of illegal drugs, dangerous gang activity, transnational criminal organizations and illegal immigration flow across our border," she said.Border security is national security, Nielsen said."We do hope that the deployment begins immediately," she said, adding the troops would support the mission in conjunction of the needs of the specific border state.DOD SUPPORT TO DHS MISSIONThe Defense Department has long supported DHS efforts to protect the nation's borders, Nielsen said, including counternarcotics missions, infrastructure construction, persistent surveillance operations training and aerial support throughout the Western Hemisphere.At the White House yesterday, Trump said he wants to use the military to secure the U.S. border with Mexico, in response to gaps in security there. "Until we can have a wall and proper security, we're going to be guarding our border with the military," he said.Previous presidents have deployed National Guard troops to support security along the U.S. southern border; 6,000 were sent under President George W. Bush, and 1,200 under President Barack Obama.