South Korea on Wednesday fired 20 machine gun warning shots after a North Korean drone briefly crossed the rivals' border, South Korean military officials said.

It is the first time shots have been fired in what has so far been a Cold War-style standoff between the Koreas in the wake of the North's nuclear test one week ago.

The North Korean drone was flying dozens of metres south of the border and turned back to the North after the South fired, South Korean defence and military officials said, requesting anonymity because of office rules. The shots didn't 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, North Korean leader Kim Jong-un 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 they were still considered a potential new security threat.

Animosity is high after the nuclear test, the North's fourth.

South Korea President North Korea Nuclear

South Korean army soldiers search for suspected North Korean leaflets on a field in Paju, near the border with North Korea. (Roh Seung-hyuck/Yonhap via AP)

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.

South Korea's president earlier Wednesday urged North Korea's only major ally, China, to help punish Pyongyang's nuclear test with the strongest possible international sanctions.

Seoul also said that North Korea had flown leaflets across the border describing President Park Geun-hye and her government as "mad dogs."

South Korea, the United States and others are pushing hard to impose fresh sanctions and other punitive measures on the North for what Pyongyang said was a hydrogen bomb test.

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.

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, is seen as key.

South Korea PM Park says China role key

On Wednesday, Park said in a nationally televised news conference that South Korea will push as hard as it can for strong sanctions that can force change in North Korea. But, she said, Chinese help is crucial.

"Holding the hands of someone in a difficult situation is the mark of the best partner," Park said, referring to China and South Korea's need to punish the North. "I trust China, as a permanent member of the Security Council, will play a necessary role."

Beijing has recently shown signs that it's losing patience with North Korea over its repeated provocations. But China is still seen as reluctant to clamp down on the North in part because of fears that a toppled government in Pyongyang would see millions of desperate North Koreans flooding across the border with China and a U.S.-backed South Korean government in control of the Korean Peninsula.

Park Geun-hye

Park Geun-hye said Wednesday the loudspeaker campaign will continue until further notice, the South Korean president describing it as an effective psychological tool. (Kim Hong-Ji/Reuters)

Responding to the North's test, U.S. Secretary of State John Kerry has urged China to end "business as usual" with North Korea. But in a telephone conversation with his South Korean counterpart last Friday, Chinese Foreign Minister Wang Yi made it clear that Beijing supports dialogue to resolve the nuclear standoff. His reported remarks sparked speculation in Seoul that China has no intention of joining in any harsh punishment on the North.

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

Park said past broadcasts helped frontline North Korean soldiers learn the truth about Pyongyang's authoritarian rule and defect to South Korea. "The most powerful threat to totalitarianism is the power of truth," she said.

South Korea's military announced Wednesday that it has found hundreds of anti-Seoul leaflets near the western portion of the Koreas' border. The defence Ministry believes the leaflets were floated over with balloons by the North's military.

Similar North Korean propaganda leaflets were discovered on a South Korea border island between late 2013 and early 2014. Such leafleting, however, by the North is still rare, though South Korean activists occasionally send anti-Pyongyang leaflets in balloons across the border.

The leaflets found earlier Wednesday included such messages as "Let's knock down the Park Geun-hye group like we do mad dogs" and "The U.S. must immediately stop its anachronistic hostile policy on North Korea."