(RNS) North Korea has detained a U.S. citizen — a self-described Christian missionary — raising the number of Americans held there to four.

Kim Hak-song, who was taken into custody Saturday (May 6), taught at Pyongyang University of Science and Technology. The university, founded in 2010 with donations from South Korean and American Christians, including the Illinois-based Church of the Brethren, remains the only private university in the country.

“Obviously, this is concerning. We’re well aware of it and we’re going to work through the embassy of Sweden … through our State Department, to seek the release of the individuals there,” White House spokesman Sean Spicer told reporters Monday.

Song was detained on suspicion of “hostile acts” against the state, the North’s KCNA news agency said.

One other of the four Americans detained, Kim Sang-duk, also taught at PUST.

Experts on North Korea consider the detentions an attempt by the authoritarian North Korean regime to gain bargaining chips as it resists U.S. pressure to rein in its nuclear program, with which it has threatened South Korea and the U.S.

(Reuters contributed to this report)