SEOUL/WASHINGTON—North Korea used what would be its largest ever intercontinental ballistic missile (ICBM) system in two recent launches, and appears to be restoring some tunnels at its shuttered nuclear test site, U.S. and South Korean officials said on Friday. The reports are the latest to suggest the country may soon follow through on threats to resume testing long-range ICBMs or nuclear weapons for the first time since 2017. The escalation in North Korea tensions comes as South Korea on Wednesday elected a new conservative president. Yoon Suk-yeol has said that pre-emptive strikes may be needed to counter any imminent attack by the North and has vowed to buy American THAAD missile interceptors, while remaining open to restarting stalled denuclearization talks. Reclusive North Korea and the affluent, democratic South are technically still at war because their 1950–53 conflict ended in a ceasefire, not a peace treaty. South Korea’s military said on …