The head of the U.N.’s nuclear watchdog expressed “deep concerns” about North Korea’s expanding nuclear weapons arsenal in a meeting with South Korea’s foreign minister on Wednesday, reiterating the need for diplomatic engagement with Pyongyang to address the challenge.

In a social media post following his meeting with South Korean Foreign Minister Cho Hyun in Seoul, International Atomic Energy Agency (IAEA) Director-General Rafael Grossi emphasized his agency’s “indispensable verification role” in line with U.N. Security Council resolutions seeking to halt Pyongyang’s nuclear development.

Cho highlighted the South Korean government’s ongoing efforts to “end hostility and confrontation between the two Koreas