WASHINGTON—The Obama administration lashed out at North Korea and called for the immediate release of an American citizen sentenced by Pyongyang this week to 15 years of hard labor for allegedly committing "hostile acts" against the isolated communist country.

The fate of Kenneth Bae, a 44-year-old Korean-American tour operator from Washington state, emerged as the latest test for the U.S. in its efforts to deal with Pyongyang and its young leader, Kim Jong Eun. North Korea has begun a string of provocative acts...