U.S. Deputy Secretary of State Wendy Sherman on Tuesday warned of a forceful response if North Korea carries out its first nuclear test explosion in nearly five years as she traveled to Seoul to meet with South Korean and Japanese allies and discuss the escalating standoff.

U.S. and South Korean officials have said North Korea is all but ready to conduct another detonation at its nuclear testing ground in the northeastern town of Punggye-ri, which last hosted a test in September 2017, when it claimed to have detonated a thermonuclear bomb designed for its intercontinental ballistic missiles.