Some 260 balloons loaded with garbage, feces and debris were sent from North Korea to South Korea, crossing the border between the two countries.
According to The Washington Post, the balloons, which began landing on Tuesday night, are Pyongyang's response to the balloons carrying propaganda, food and money that South Korean activists have sent to the North in recent years.
The South Korean General Staff has sent out disaster alerts to residents, warning them to “refrain from outdoor activities” and not to come into contact with the balloons and to report them to the police or military.
“These North Korean acts are clear violations of international law and seriously threaten the security of our citizens,” the General Staff said in a statement.
The balloons were carrying garbage, including plastic bottles and batteries, as well as what the General Staff called “dirt”.
According to the news agency, one balloon that landed on a restaurant building in Dongducheon was carrying a bag of manure.
The military also said that any damage caused by the balloons would be the sole responsibility of North Korea and demanded that Pyongyang put an end to “these inhumane and despicable actions”.
On Sunday, North Korea said it would take “retaliatory action” against South Korean propaganda operations by spreading “piles of garbage and dirt” over the border.
“South Korea will directly experience the effort needed to remove them,” said North Korea. “When our sovereignty, security and national interests are violated, we will act immediately.”
In recent years, South Korean activists, including defectors from the North, have sent thousands of balloons and plastic bottles with food, money and propaganda to the North.
According to the South Korean General Staff, in 2016, balloons sent by the North caused damage to a vehicle and the roof of a house. At the time, the balloons reportedly carried toilet paper, cigarette butts and garbage, which the Seoul police classified as “dangerous biochemical substances”.
The North has long been irritated by the balloons, which fuel tensions between the two sides. In 2015, Pyongyang announced that it would open fire on activists' balloons carrying propaganda.
The two countries, which signed an armistice in 1953 but never officially signed a peace treaty to end the Korean War, are separated by a fortified demilitarized zone, and remain technically at war.