Seoul, South Korea - North Korea yet again sent balloons carrying trash across the heavily-guarded border into South Korea, the Yonhap news agency reported on Wednesday.

At least one of the balloons had landed on the grounds of the South Korean presidential office, Yonhap reported, citing the Presidential Security Service (PSS).

"An investigation by the chemical, biological and radiological response team showed the objects did not present a danger or contamination, so they were retrieved," the PSS said.

According to Yonhap, the South Korean Joint Chiefs of Staff (JCS) said that a number of balloons floated across the border around Seoul and the surrounding province. The JCS advised people not to touch the objects but to report them to authorities.

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Since the end of May, North Korea has sent more than 3,000 trash-filled balloons into the South, including some 500 on Sunday.

North Korea has been engaged in a chaotic tit-for-tat with South Korean activists, who repeatedly send thousands of leaflets and other anti-regime propaganda material across the border in huge gas balloons.

Seoul has also ramped up propaganda broadcasts, which infuriate Pyongyang.

Tensions have been on the rise again along the demilitarized zone and the maritime border that separate the two Korean states, who are still officially at war since the civil war of the 1950s.