WASHINGTON (AP) — The U.S. is closely watching North Korea for signs of a possible missile launch or nuclear test in the coming days that officials are referring to as a "Christmas surprise."

A significant launch or test would mean the end of North Korea's self-imposed moratorium and raise tensions in the region. It would also be a major blow to one of the Trump administration's major foreign policy initiatives: the drive to get North Korea back to negotiations to eliminate its nuclear weapons and missiles.

Earlier this month, the North conducted what U.S. officials say was an engine test. Nor...