SEOUL, South Korea — 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 drone was flying dozens of yards south of the border and turned back after the South fired the shots, South Korean 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. Animosity has been high since the North’s claim on Jan. 6 that it 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.

Seoul said Wednesday that thousands of North Korean leaflets, likely sent across the border by balloons, have been found in the South. Some described South Korean President Park Geun-hye and her government as “mad dogs.”

North Korea’s propaganda machine is using the nuclear test to glorify Kim’s leadership.

Park said that South Korea will continue its loudspeaker campaign, calling it “the surest and most effective psychological warfare tool.”

She said past broadcasts helped front-line North Korean soldiers learn the truth about Pyongyang’s authoritarian rule.