SEOUL — North Korea on Thursday threatened a preemptive nuclear strike against the United States and other purported aggressors, describing Washington as a “criminal threatening global peace.” Though Pyongyang routinely vows to demolish the United States in a sacred war, the threat issued Thursday marked a major escalation of rhetoric just hours before the United Nations Security Council is to discuss new sanctions aiming at reining in the North’s weapons program and restricting illicit overseas trade.