The targeted killing of those suspected of engaging in terrorist activities against the United States, including American citizens, is justified and legal, according to the Defense Department's chief lawyer.

Pentagon general counsel Jeh Johnson is the first government lawyer to officially weigh in on the legal justification for killing a U.S. citizen since American born Yemeni cleric Anwar al-Awlaki was killed by a CIA missile fired from an unmanned aerial vehicle last September.

In comments Wednesday night during a speech at Yale University, Johnson made no mention by name of al-Awlaki or the classified CIA drone program.

"Belligerents who also happen to be U.S. citizens do not enjoy immunity where non-citizen belligerents are valid military objectives," Johnson said.

He cited a 2004 Supreme Court decision as the justification for his comment.