SEOUL, South Korea (AP) — South Korean soldiers fired warning shots after North Korean troops violated the land border earlier this week, South Korea's military said Tuesday.

Some North Korean soldiers who were engaged in unspecified work on the northern side of the border briefly crossed the military demarcation line on Sunday, South Korea’s Joint Chiefs of Staff said.

Those North Korean soldiers returned to their territory after South Korea’s military fired warning shots and issued warning broadcasts, the Joint Chiefs of Staff said.

It said North Korea had not conducted any other suspicious activities.

The Koreas’ mine-strewn land border is the world’s most heavily armed border, with hundreds of thousands of combat troops facing each other. It’s a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty.

The border-crossing incident came amid rising tensions over North Korea’s recent launches of trash-carrying balloons.