Trash balloons floated by North Korea are spotted in skies above Seoul on Monday. (Yonhap)

South Korea's military on Monday vowed to take "stern" military measures should North Korea "cross the line" with its ongoing trash balloon campaign or inflict serious damage to the South Korean people.

The Joint Chiefs of Staff made the remark in a statement in response to the North's repeated launch of trash-carrying balloons across the border, including those attached with timer devices that could potentially cause fires.

Since late May, the North has launched some 5,500 balloons carrying trash on over 22 occasions in retaliation against anti-Pyongyang leaflets sent across the border by North Korean defectors and activists in South Korea.

The North floated some 120 balloons from Sunday evening to Monday morning in its latest launch. No additional balloons were spotted midair, the JCS said in a notice sent to reporters at 9:14 a.m.

"While there may be inconveniences and difficulties caused by North Korea's trash balloons, our fundamental measure to eradicate them is to show that 'there is nothing to gain for the enemy,'" the JCS said in the statement.

"Still, if serious danger is caused to the safety of our citizens or if the North is assessed to have crossed the line, our military will take stern military measures," it added, without elaborating on what the threshold will be.

The military, which has refrained from directly shooting down the balloons, ruled out views calling for such action, saying that they could lead to bigger issues for the public's safety.

The JCS condemned the North's balloon campaign as not only a "globally embarrassing" act but as a "vulgar" action aimed at stirring up conflict within the South.

In response to the balloon launches, the South's military has been blasting daily anti-North Korean propaganda broadcasts through loudspeakers on the border since July 21. (Yonhap)