South Korea has fired 20 machine gun warning shots after a North Korean drone briefly crossed its border, officials said.
The shots are the first to be fired in a Cold War-style stand-off between the Koreas in the wake of the North's nuclear test last week.
The North Korean drone was flying dozens of metres south of the border and turned back to the North after the South fired shots, South Korean defence and military officials said. 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 programme, 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 January 6 that it had tested a hydrogen bomb. There is widespread scepticism over the H-bomb claim, but whatever the North detonated underground will likely push the country closer towards 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 that North Korea had also flown balloons with thousands of leaflets across the border, some of them describing 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 Jong Un's leadership and describing it as a necessary step to fight against what it calls a US-led attempt to overthrow the North's system.
Pyongyang's state TV aired photos showing a smiling Kim Jong Un awarding commendations to scientists and other workers involved in the test and shaking hands with them at a Workers' Party building. He called the scientists and workers "nuclear combatants" and "the heroes of heroes" who evoked fears to the US and its allies, according to the state news agency.
Ms Park urged North Korea's only major ally, China, to help punish Pyongyang for its nuclear test with what she called "the strongest" possible international sanctions that can force change in the North.
Diplomats at a UN Security Council emergency session last week pledged to swiftly pursue new sanctions. For current sanctions and any new penalties to work, better cooperation and stronger implementation from China, the North's diplomatic and economic protector and a veto-wielding permanent member of the Security Council, are seen as key.
"Holding the hands of someone in a difficult situation is the mark of the best partner," Ms Park said at a news conference, referring to China and South Korea's need to punish the North. "I trust that China, as a permanent member of the Security Council, will play a necessary role."