SEOUL, South Korea — South Korea today fired 20 machine gun warning shots after a North Korean drone briefly crossed the rivals’ border, officials said, the first shots fired in a Cold War-style standoff between the Koreas in the wake of the North’s nuclear test last week.

The North Korean drone was flying dozens of meters (yards) south of the border and turned back to the North after the South fired the shots, South Korean defense and military officials said, requesting anonymity because of office rules. The shots did not hit the drone.

North Korean drone flights across the world’s most heavily armed border are rare, but have happened before.

North Korea has in recent years touted its drone program, a relatively new addition to its arsenal. In 2013, state media said North Korean leader Kim Jong Un had watched a drone attack drill on a simulated South Korean target.

In 2014, Seoul officials discovered what they called several North Korean drones that had flown across the border. Those drones were crude and decidedly low-tech, but were still considered a potential new security threat.