South Korea’s Vice Foreign Minister Park Yoon-joo lodged a formal complaint with Ukraine’s acting ambassador, Andrii Vieshkin, and expressed “strong regret” that President Volodymyr Zelensky revealed the transfer of two captured North Korean soldiers, which Seoul says was supposed to be a secret, according to the BBC.

Seoul summoned the diplomat after Zelensky announced last week that his government had transferred the two soldiers to Seoul, according to the BBC.