South Korea’s Joint Chiefs of Staff (JCS) said on Sunday that warning shots had been fired when North Korean soldiers violated the military demarcation line between the countries, prompting them to return.

The Yonhap news agency reported earlier on Sunday that warning shots had been fired last week, citing an unnamed source.

“Recently, on the Eastern Front, North Korean forces crossed the military demarcation line, prompting our military to fire warning shots according to operational procedures,” a JCS statement said without giving the date of the incident.

“Our military is closely monitoring North Korean movements while maintaining a firm military readiness posture.”

Yonhap said the incident was the first reported violation of the military demarcation line by North Korea this year and that the soldiers were likely to have been patrolling the area when they crossed the MDL.

The North Korean embassy in Singapore did not respond immediately to a request for comment.

North Korean troops violated the line 17 times last year, Yonhap said.

The two Koreas are technically at war after their 1950-53 war ended in an armistice rather than a peace treaty.