North Korea and South Korea on Friday exchanged machine-gun fire after the North shot down balloons containing anti-government propaganda released by activists in the South, Seoul’s military said, according to reports. The South Korean military said that it reacted after some of the shots fired by the North had landed across the border.

The shooting continued for two hours after a group of South Korean activists flew 200,000 balloons containing propaganda leaflets, Yonhap, a South Korean news agency reported. South Korea’s military reportedly retaliated with nearly 40 rounds after a warning fired 10 minutes earlier. On Thursday, the North had warned Seoul to stop the leaflet campaign or it would face an "uncontrollable catastrophe," according to Yonhap. Immediate details about casualties on either side were not available. 

On Tuesday, both countries had fired warning shots against each other when a North Korean patrol ship crossed a disputed region.

In recent weeks, Pyongyang has been on edge after warning the South not to conduct joint military drills with the U.S. The North also extended the warning and threatened to attack the U.S. with nuclear weapons if it continued with the drills, which both the U.S. and South Korea have termed a regular military exercise.

The incident with the balloons occurred on the founding anniversary of the North’s ruling Workers’ Party of Korea. The celebration arranged for the anniversary in Pyongyang was not attended by the North’s leader Kim Jong Un, who is suspected to be ill.