Trash carried by a North Korean balloon fell on the presidential compound in central Seoul on Thursday in a second case raising concerns about the vulnerability of key South Korean sites during potential North Korean aggression.

The incident comes after the rival Koreas ramped up threats and rhetoric against each other over North Korea's claims that South Korea flew drones over its capital Pyongyang to scatter propaganda leaflets this month.

No dangerous items were found in the rubbish that was dropped on the ground when one of the North Korean balloons burst over the South Korean presidential compound on Thursday morning, South Korea's presidential security service said in a statement.

North Korea has sent thousands of balloons carrying bags of rubbish like plastic and paper waste into South Korea since late May in a resumption of a Cold War-style psychological campaign. The trash that fell on the South Korean presidential compound in July contained no hazardous materials as well.

It wasn't immediately known whether South Korean President Yoon Suk Yeol was at the compound during the latest incident. Later Thursday, he met visiting Polish President Andrzej Duda at his office.

South Korean media reported that North Korean leaflets criticising Yoon and his wife Kim Keon Hee were found on Thursday in Seoul's Yongsan district, where Yoon's presidential office is located. Media published photos of some leaflets that described Kim as a latter-day Marie Antoinette, the queen who was beheaded in 1793 during the French Revolution.

The reports said it was the first time that North Korean propaganda leaflets have been found in South Korea since the North began its balloon campaign five months ago.

The South Korean presidential security service didn't immediately confirm the report.

Experts say North Korea likely lacks sophisticated technology to drop balloons on specific targets.

“Whether the balloons have GPS or not, it's all about launching them in large numbers and hitting the right altitude based on wind direction and speed, so that they can ride those winds to travel,” Lee Choon Geun, an honorary research fellow at South Korea's Science and Technology Policy Institute, said.

“While some media are saying the accuracy of the balloons has improved, that improved accuracy isn't because they equipped them with some sort of guidance system, but rather because it's the season when winds blow southward," Lee said.

North Korea has earlier accused South Korea of infiltrating drones to drop propaganda leaflets over Pyongyang three times this month and threatened military responses if it happened again. South Korea has refused to confirm whether it sent drones but warned that North Korea would face the end of its regime if the safety of South Korean citizens is threatened.

North Korea said its balloon activities were a tit-for-tat action against South Korean activists launching anti-Pyongyang leaflets via their own balloons.

South Korea responded by restarting propaganda loudspeaker broadcasts at border areas, prompting North Korea to turn on their own frontline loudspeakers.

The Koreas' Cold War-style campaigns come as North Korean leader Kim Jong Un has increased the pace of his weapons tests and expanded military cooperation with Russia.

US and South Korean officials said Wednesday that 3,000 North Korean troops have been deployed to Russia and are training at several locations. South Korean officials say North Korea eventually aims to send a total of 10,000 troops to Russia to support its war efforts in Ukraine.

South Korea is concerned that Russia may reward North Korea by giving it sophisticated technologies that could improve the North's nuclear and missile programs that target South Korea and the United States.