North Korea’s UN ambassador criticized U.S. plans to go forward with joint military drills with South Korea this week, saying they could lead to a “second Korean War” and hinting at nuclear “countermeasures.”

Speaking in Geneva, So Se Pyong called the annual drills an “open challenge” following North Korea’s offer, made at the end of last week, to end its nuclear tests in exchange for the direct talks and the halting of U.S.-ROK drills.

The U.S. has rejected the North’s offer, stating that the drills are “transparent” and defensive in nature and that Pyongyang cannot offer to halt nuclear testing,