Seoul, South Korea - North Korea has floated more trash-filled balloons southward, Seoul's military said Thursday, the latest in a series of border barrages that have sparked a tit-for-tat propaganda campaign.

Pyongyang launched about 420 balloons late Wednesday, quickly followed by another round early Thursday, Seoul's Joint Chiefs of Staff (JCS) said.

Around 20 of the balloons had already landed in the South, mainly in northern Gyeonggi province and the capital Seoul, the JCS said.

The bags attached to those balloons contained "mostly paper and plastic waste", the military said, adding that they posed no safety risk to the public, according to their analysis.

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This is the thirteenth round of trash-carrying balloons launched by Pyongyang since late May.

The latest launch comes as relations between the two Koreas are at one of their lowest points in years, with the North recently announcing the deployment of 250 ballistic missile launchers to its southern border and testing new weapons.

North Korea has sent more than 3,800 trash-filled balloons southward since May, saying they are retaliation for propaganda balloons launched by South Korean activists.

In response, Seoul has suspended a tension-reducing military deal with Pyongyang and restarted some propaganda broadcasts from loudspeakers along the border.

South Korean officials held high-level talks with US officials on North Korea deterrence on Wednesday.

"North Korea has not stopped advancing its nuclear and missile capabilities and has recently continued to provoke by disturbing the GPS system or launching trash balloons," Kim Hong-kyun, South Korea's vice minister of foreign affairs, told reporters.

"In this situation, the two countries cannot rule out the possibility of North Korea making a major provocation before and after the US presidential election," he added.