Russia launched multiple North Korean ballistic missiles into Ukraine in the past week, the U.S. announced on Thursday, attacks that if confirmed would represent a major escalation in weapons cooperation between Pyongyang and Moscow and likely the first-ever use of DPRK ballistic missiles in combat.

Moscow’s forces attacked Ukraine with at least one ballistic missile on Saturday and several more as part of an aerial attack on Tuesday, according to John Kirby, the spokesperson of the White House’s National Security Council (NSC).