President Obama announced in March that he would be sending 21,000 additional troops to Afghanistan. But in an unannounced move, the White House has also authorized -- and the Pentagon is deploying -- at least 13,000 troops beyond that number, according to defense officials.

The additional troops are primarily support forces, including engineers, medical personnel, intelligence experts and military police. Their deployment has received little mention by officials at the Pentagon and the White House, who have spoken more publicly about the combat troops who have been sent to Afghanistan.