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.

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

Seoul also said that North Korea had flown leaflets across the border describing its president and her government as "mad dogs.”

The incidents came amid tension after what North Korea claimed was a nuclear test, which, if verified, would be the North's fourth.

Earlier Wednesday, 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.

On Wednesday, President Park Geun-hye 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."

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.

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.

The Associated Press