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.

Axar.az reports that 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.

In late April, the Swedish Embassy in Pyongyang confirmed that a Korean-American man in his fifties was taken into detention by North Korean police when he was trying to leave the country.

The detentions are expected to further fuel tensions between North Korea and the US. The two countries have been engaged in a heated standoff over Pyongyang’s missile and nuclear programs.

The US has dispatched a military strike group to the Korean Peninsula. Pyongyang has said it is ready for war.