South Korea Wednesday fired warning shots after an unidentified object from North Korea was seen flying close to the rivals’ border, the South’s military said.

Media reported that it was a drone. The incident comes amid a deepening standoff between the Koreas in the wake of the North’s nuclear test one week ago.

The North Korean object turned around after the South fired the shots, the South’s Joint Chiefs of Staff said in a statement. It did not say whether the South Koreans hit the object. Yonhap news agency reported that the South fired 20 rounds from machine guns at a drone. North Korean drone flights across the border are rare but do occasionally happen across the world’s most heavily armed border. Animosity is high after the nuclear test, the North’s fourth.

South Korea’s President earlier Wedneday urged North Korea’s only major ally, China, to help punish Pyongyang’s recent nuclear test with the strongest possible international sanctions.

Seoul also said that North Korea had flown leaflets across the border describing her and her government as “mad dogs” as Cold War-style propaganda warfare continued between the rivals.

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.

US House Republicans and Democrats joined together on Tuesday to overwhelmingly approve legislation that aims to punish North Korea for conducting its latest nuclear test.