North Korea has detained another US citizen on suspicion of “hostile acts” against the state, 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 third US citizen, Kim Sang Dok, who was associated with the same school, was detained in late April for ‘hostile acts’, according to North Korea’s official media.

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

“The security of US citizens is one of the department’s highest priorities.

When a US citizen is reported to be detained in North Korea, we work with the Swedish embassy in Pyongyang,” a State Department official said in a 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 US-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.

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.

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.

– Reuters