The White House secretly sent a diplomatic mission to North Korea this week to recover a U.S. citizen who the Trump administration had learned was in a life-threatening coma, senior U.S. officials said, an episode that could inflame tensions with Pyongyang.

The freed American, University of Virginia student Otto Warmbier, was sentenced early last year to 15 years of hard labor for allegedly defacing a political poster while on a private tour of North Korea. He arrived late Tuesday in Cincinnati, where his plane was met by...