South Korea has threatened to restart anti-Pyongyang frontline propaganda broadcasts in the latest bout of Cold War-style campaigns between the rivals after North Korea resumed its rubbish-carrying balloon launches.

On Monday night, North Korea floated huge balloons carrying plastic bags of rubbish across the border in its fifth such campaign since late May – an apparent response to South Korean activists flying political leaflets via balloons.

South Korean President Yoon Suk Yeol called North Korea’s balloon activities “a despicable and irrational provocation”.

In a speech marking the 74th anniversary of the start of the 1950-53 Korean War, Mr Yoon said on Tuesday that South Korea will maintain a firm military readiness to overwhelmingly respond to any provocations by North Korea.

South Korea’s military said North Korea floated about 350 balloons in its latest campaign, and about 100 of them eventually landed on South Korean soil, mostly in Seoul and nearby areas. Seoul is about 25-30 miles (40-50km) from the border.

The military said the rubbish carried by the North Korean balloons was mostly papers and that no hazardous items were found.

In its earlier balloon launches, North Korea dropped manure, cigarette butts and waste batteries along with cloth scraps and waste papers in various parts of South Korea. No major damage was reported.

In response, on June 9 South Korea redeployed gigantic loudspeakers along the border for the first time in six years and briefly resumed anti-North Korean propaganda broadcasts.

Joint Chiefs of Staff spokesman Lee Sung Joon told reporters on Tuesday that the South Korean military is ready to turn on its border loudspeakers again.

A written Joint Chiefs of Staff statement said officials would examine unspecified strategic operational circumstances and that resumption of broadcasts would depend on how North Korea acts.

Balloon launches and loudspeaker broadcasts were psychological campaigns that the two Koreas specialised in during the Cold War. The rivals have agreed to halt such activities in recent years, but occasionally resumed them when animosities rekindled.

North Korea is highly sensitive to South Korean border broadcasts and civilian leafleting campaigns as it bans most of its 26 million people from accessing foreign news.

South Korean leafleting campaigns by civilian activists, mostly North Korean defectors, include leaflets critical of North Korea’s human rights violations and USB sticks containing South Korean TV dramas, while the past South Korean border broadcasts included K-pop songs, weather forecasts and outside news.

In a statement on Friday, Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, called them “human scum” and “disgusting defectors”.

South Korean officials maintained they do not restrict activists from flying leaflets to North Korea in line with a 2023 constitutional court ruling that struck down a law criminalising such leafleting, calling it a violation of free speech.

Many experts say the North Korean balloon campaign is also likely to be designed to deepen a debate in South Korea over the civilian leafleting and trigger a broader internal divide.

Worries about North Korea intensified in mid-June, when North Korean leader Kim Jong Un and Russian President Vladimir Putin signed a deal requiring each country to provide aid if attacked and vowed to boost other co-operation.

Observers say the accord represents the strongest connection between the two countries since the end of the Cold War.

The United States and its partners believe North Korea has been providing Russia with much-needed conventional arms for its war in Ukraine in return for military and economic assistance.

In his Korean War speech, Mr Yoon described the Kim-Putin deal as “anachronistic”.

South Korea, the US and Japan issued a joint statement on Monday strongly condemning expanding military co-operation between Russia and North Korea.