North Korea has detained a South Korean worker at the Gaeseong Industrial Complex.

The Unification Ministry said Pyongyang notified that it was detaining a South Korean worker for investigation. In a message sent on Monday afternoon, the North Korean immigration office at the inter-Korean joint industrial complex claimed the South Korean worker criticized the North’s political system.

According to the ministry, the message said the North will ensure the health, safety and human rights of the worker in accordance with inter-Korean agreements on the Gaeseong Industrial Complex.

The ministry did not disclose the identity of the detained worker.