By Ju-min Park

SEOUL (Reuters) - North Korea said on Sunday it has detained another U.S. citizen on suspicion of "hostile acts" against the state, which would make him the fourth American to be held by the isolated country amid heightened diplomatic tensions with Washington.

Kim Hak Song, who worked for the Pyongyang University of Science and Technology, was detained on Saturday, the North's KCNA news agency said.

"A relevant institution of the DPRK detained American citizen Kim Hak Song on May 6 under a law of the DPRK on suspension of his hostile acts against it," KCNA said. DPRK is short for the Democratic People's Republic of Korea, North Korea's official name.

A third U.S. citizen, Kim Sang Dok, who was associated with the same school, was detained in late April for hostile acts, according to the North's official media.

The U.S. State Department said it is aware of the latest reported detention.

Related:

Watch Today Show on Yahoo View, available now on iOS and Android.

"The security of U.S. citizens is one of the department's highest priorities. When a U.S. citizen is reported to be detained in North Korea, we work with the Swedish embassy in Pyongyang," a State Department official said in an emailed statement, declining to provide further details for privacy reasons.

The reported detention comes as tensions on the Korean peninsula run high, driven by harsh rhetoric from Pyongyang and Washington over the North's pursuit of nuclear weapons in response to what it says is a threat of U.S.-instigated war.

The Pyongyang University of Science and Technology (PUST) was founded by evangelical Christians and opened in 2010. Its students are generally children of the country's elite.

The volunteer faculty of PUST, many of whom are evangelical Christians, has a curriculum that includes subjects once considered taboo in North Korea, such as capitalism. The college is an unlikely fit in a country that has been condemned by the United States for cracking down on freedom of religion.

Story continues

A message by Kim Hak Song dated February 2015 on the website of a Korean-Brazilian church in Sao Paulo said he was a Christian missionary planning to start an experimental farm at PUST and was trying to help the North Korean people learn to become self-sufficient.

No further details were available about the circumstances related to the arrests of the two men associated with the college. A spokesman for PUST was not immediately available for comment.

North Korea, which has been criticized for its human rights record, has in the past used detained Americans to extract high-profile visits from the United States, with which it has no formal diplomatic relations.

The other two Americans already held in North Korea are Otto Warmbier, a 22-year-old student, and Kim Dong Chul, a 62-year-old Korean-American missionary.

Warmbier was detained in January 2016 and sentenced to 15 years hard labor for attempting to steal a propaganda banner.

Two months later, Kim Dong Chul was sentenced to 10 years hard labor for subversion. Neither has appeared in public since being sentenced.

(Additional reporting by Sarah N. Lynch in Washington; Writing by Jack Kim; Editing by Ros Russell and Paul Simao)