A fresh storm may be brewing for U.S. President Joe Biden as he grapples with a number of foreign policy challenges, with this one arriving on the Korean Peninsula.

Nuclear-armed North Korea appears to have restarted the plutonium-producing reactor at its Yongbyon nuclear site, the U.N. atomic agency has said, calling the development ¡°deeply troubling.¡±

The apparent restart last month ¡ª which would be the first time in nearly three years that the 5-megawatt reactor has been operational ¡ª could help North Korean leader Kim Jong Un add to his growing stockpile of fissile material.