South Korea on Monday warned of possible military actions if North Korea continues to fly more trash balloons across the border, a rare direct threat of retaliation since Pyongyang started sending them in May.

The latest batch of trash-filled balloons disrupted operations at South Korea’s main Incheon International Airport for more than 90 minutes Monday and comes as tensions along the heavily-militarized Korea border have grown in recent months.

"Our military will carry out stern military measures if North Korea’s continued trash balloons seriously threaten the safety of our people or if they are deemed to have crossed the line,” South Korea’s Joint Chiefs of Staff said in a statement Monday. It declined to elaborate what measures were under consideration.