South Korean President Yoon Suk-yeol urged the chief of the U.N. to take a strong stance against North Korea’s nuclear and missile programs on Friday, warning that a disjointed response could encourage Pyongyang’s ongoing weapons development.

Yoon’s phone talks with U.N. Secretary-General António Guterres in the morning followed a failed attempt to pass a new DPRK sanctions resolution at the new U.N. Security Council last month.