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

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

As of around 10am 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 on 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. (AFP)