South Korean President Park Geun-hye answers to a reporter’s question during her news conference at the Presidential Blue House in Seoul, South Korea, on Wednesday.

South Korea on Wednesday 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 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.

Animosity has been high since the North’s claim on Jan. 6 that it had tested a hydrogen bomb. There is widespread skepticism over the H-bomb claim, but whatever the North detonated underground will likely push the country closer toward a fully functional nuclear arsenal, which it still is not thought to have. The North previously conducted atomic bomb tests in 2006, 2009 and 2013.

Since Friday, South Korea has been blasting anti-Pyongyang propaganda and K-pop songs from huge speakers along the border. The North, which calls the broadcasts an act of war meant to threaten its system of government, is using speakers of its own in an attempt to keep its soldiers from hearing the South Korean messages.

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