North Korea says it has arrested yet another US citizen on charges of acting against its sovereignty, making him the fourth American national to be detained by Pyongyang over the past weeks.

Kim Hak Song, who worked for the Pyongyang University of Science and Technology, was arrested on May 6, North Korea’s official news agency KCNA reported Sunday.

“A relevant institution of the DPRK [Democratic People's Republic of Korea] detained American citizen Kim Hak Song on May 6 under a law of the DPRK on suspension of his hostile acts against it,” the report said.

The university that Song worked for was founded by evangelical Christians in 2010 and its students are mostly children of the elite families.

In late April, the Swedish Embassy in Pyongyang confirmed that Kim Sang Dok, a Korean American in his fifties, was taken into detention by North Korean police when he was trying to leave the country. He was also associated with the same school.

There were no further details available about the arrests. The university has yet to react.

There are no diplomatic relations between the US and North Korea. The new arrest is expected to further fuel tensions between the two countries, which have been engaged in a heated standoff over Pyongyang’s testing of ballistic missiles and nuclear deterrents.

The US has dispatched a military strike group to Korean waters in order to hold military drills with Japan and South Korea. Pyongyang has conducted a live artillery firing drill in response, warning the US and its regional allies against any invasion.

The North had previously arrested US citizens Otto Warmbier, a 22-year-old student, and Kim Dong Chul, a 62-year-old missionary.

Warmbier was detained in January 2016 and sentenced to 15 years of hard labor for attempting to steal a banner. Kim Dong Chul was sentenced to 10 years hard labor for spying.

Over the past decade, North Korea has arrested and jailed several US citizens and later released them, following high-profile visits by current or former US officials.