Trump picks retired Marine Gen. John F. Kelly to head Department of Homeland Security

NEW YORK/WASHINGTON, Dec 7 (Reuters) - President-elect Donald Trump plans to nominate a third former general for a top job in his new administration with the choice of retired Marine General John Kelly to lead the Department of Homeland Security.

A source familiar with the situation told Reuters on Wednesday that Trump was expected to name Kelly, 66, to head the agency, which oversees border security, but did not elaborate.

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Kelly is the former head of the military's Southern Command, where he was responsible for U.S. military activities and relationships in Latin America and the Caribbean.

Kelly retired in January, 2016 after a 45-year military career.

The Republican president-elect, who has no military experience, also plans to nominate retired General James Mattis to lead the Department of Defense and picked retired Lieutenant General Michael Flynn to be his national security adviser.

Although Kelly's military experience may give him insight into overseas threats like drug trafficking or Islamist militancy, he would face new challenges at an agency that oversees everything from airport security to protecting against cyber threats and responding to domestic security crises.

In 2015 testimony before the Senate Armed Services Committee, Kelly warned of potentially dire consequences from illegal immigration on the U.S.-Mexico border.

"The relative ease with which human smugglers moved tens of thousands of people to our nation's doorstep also serves as another warning sign: These smuggling routes are a potential vulnerability to our homeland," Kelly told U.S. lawmakers.

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"Terrorist organizations could seek to leverage those same smuggling routes to move operatives with intent to cause grave harm to our citizens or even bring weapons of mass destruction into the United States," he said.

Such sentiments may have endeared him to Trump, who warned repeatedly during his presidential campaign of dangers from immigration, and pledged to build a wall along the border and make Mexico pay for it.

Kelly also questioned the Pentagon's decision to allow women to serve in combat, and differed publicly with President Barack Obama over the Democrat's attempt to close the Guantanamo Bay prison in Cuba.

"There are no innocent men down there," Kelly said of Guantanamo in a January interview with the Military Times newspaper.

The Republican-controlled Senate must confirm Kelly for the Homeland Security post.