In this file photo, taken on Jul. 24, a North Korean balloon carrying trash flies in the air. (Newsis)

North Korea again started floating balloons into South Korea on Thursday morning, South Korea's military said, hours after it sent hundreds of balloons across the border the previous day.

"As North Korea has again been floating balloons since 9 a.m., we advise the public to beware of falling objects and report to nearby military units or the police when discovering a fallen balloon," the Joint Chiefs of Staff (JCS) said in a notice to reporters.

The move came after North Korea sent around 420 trash-carrying balloons from Wednesday night to early Thursday in what marked its resumption of its balloon campaign after a near monthlong halt.

Around 20 balloons were retrieved in Seoul and the surrounding Gyeonggi Province, with most of them containing scrap paper and plastic bottles, the JCS said. No hazardous substances have been found.

Since late May, the North has launched thousands of balloons carrying trash in a tit-for-tat move against anti-Pyongyang leaflets sent by North Korean defectors and activists in South Korea. Pyongyang last launched such balloons on Aug. 10.

In response to the North's campaign, South Korea's military began blasting anti-Pyongyang propaganda broadcasts daily through loudspeakers on the border in July.

North Korea has bristled against the anti-Pyongyang leaflet and loudspeaker campaigns on fears that an influx of outside information could pose a threat to the Kim Jong-un regime. (Yonhap)