North Korea has sent around 600 more trash-filled balloons containing everything from cigarette butts to plastic across the border, Seoul's military said Sunday, adding that security personnel were collecting them as they landed.

"North Korea has resumed launching waste balloons towards South Korea," since around 8 pm (1100 GMT) Saturday evening, Seoul's Joint Chiefs of Staff said in a statement.

As of around 10 am Sunday, "approximately 600 balloons have been identified, with about 20 to 50 balloons per hour moving through the air," it added.

The balloons are landing in South Korea's northern provinces, including the capital Seoul and the adjacent area of Gyeonggi, which collectively are home to nearly half of the South's population.

North Korea began sending hundreds of balloons carrying bags of trash earlier this week, which Seoul has slammed as "low-class", warning of strong countermeasures unless Pyongyang stops such "irrational" provocations.

Since the campaign started Tuesday, some 900 balloons have been launched, the JCS said.

So far, they have been found to contain "waste such as cigarette butts, scrap paper, fabric pieces, and plastic," it said, adding that "no hazardous substances have been found."

"Our military is conducting surveillance and reconnaissance from the launch points of the balloons, tracking them through aerial reconnaissance, and collecting the fallen debris, prioritising public safety," it said.

"We urge the public to avoid contact with the fallen waste balloons and report them to the nearest military unit or police station," it added.

- Balloon wars -

The Seoul city government sent a text alert to residents on Saturday, warning of an "unidentified object presumed to be North Korean propaganda leaflets".

Pyongyang defended its release of the balloons earlier this week, saying the "sincere gifts" were retaliation for the balloons sent into North Korea with propaganda against leader Kim Jong Un.

North Korea has long been infuriated by the balloons sent by South Korean activists, which carry anti-Pyongyang leaflets. Sometimes, they also include cash, rice or USB thumb drives with South Korean drama series.

In 2018, during a period of improved inter-Korean relations, the leaders of the two Koreas agreed to "completely cease all hostile acts against each other in every domain", including the distribution of leaflets.

The South Korean parliament passed a law in 2020 criminalising the act of sending leaflets to the North, but the activists did not stop.

That same year, Pyongyang, blaming the anti-North leaflets, unilaterally cut off all official military and political communication links with the South and blew up an inter-Korean liaison office on its side of the border.

Last year, South Korea's Constitutional Court struck down the 2020 law, calling it an undue limitation on free speech.

Kim Jong Un's sister Kim Yo Jong -- one of Pyongyang's key spokespeople -- mocked South Korea for complaining about the balloons this week, saying North Koreans were simply exercising their freedom of expression.