Yukiya Amano, head of the International Atomic Energy Agency (IAEA), called North Korea’s nuclear program a “matter of serious concern” in a statement to the IAEA Board of Governors on Monday.

Amano voiced particular concern over the North’s insistence of its “right” to conduct nuclear tests and its announcements of plans to restart its nuclear facilities at Yongbyon, urging North Korea to “cooperate promptly with the Agency” and “resolve all outstanding issues.”

The North Korean government has continuously expanded its nuclear program in recent years, conducting its third nuclear test in February 2013 while reiterating its necessity as a defensive