SOUTH KOREA summoned the Russian ambassador to protest against the country’s new defence pact with North Korea today, following a seemingly accidental incursion by North Korean troops over the border.

The powerful sister of North Korean leader Kim Jong Un issued a vague threat of retaliation today morning after South Korean activists flew balloons carrying anti-Pyongyang propaganda leaflets across the border, and South Korea’s military said it had fired warning shots the previous day to repel North Korean soldiers who briefly crossed the rivals’ land border for the third time this month.

That came two days after Moscow and Pyongyang reached a pact vowing mutual defence assistance if either is attacked, and a day after Seoul responded by saying it would consider providing arms to Ukraine to fight Russia’s invasion.

South Korean Vice-Foreign Minister Kim Hong Kyun summoned Russian ambassador Georgy Zinoviev to protest against the deal between Russian President Vladimir Putin and Kim Jong Un and called for Moscow to immediately halt its alleged military co-operation with Pyongyang.

Kim Hong Kyun stressed that any co-operation that directly or indirectly helps the North build up its military capabilities would violate UN security council resolutions and pose a threat to the South’s security, and warned of consequences for Seoul’s relations with Moscow.

Mr Zinoviev told South Korean officials that any attempts to “threaten or blackmail” Russia were unacceptable and that his country’s agreement with North Korea wasn’t aimed at specific third countries, Russia’s embassy wrote on Twitter.\

Leafleting campaigns by South Korean civilian activists in recent weeks have prompted a resumption of cold war-style psychological warfare along the inter-Korean border.

The South Korean civilian activists, led by North Korean defector Park Sang Hak, said it sent 20 balloons carrying 300,000 propaganda leaflets, 5,000 USB sticks with South Korean pop songs and TV dramas, and 3,000 US dollar bills from the South Korean border town of Paju on Thursday night.

Pyongyang resents such material and fears it could demoralise front-line troops and residents and eventually weaken Kim Jong Un’s grip on power, analysts say.

In a statement carried by North Korea’s official Korean Central News Agency, Kim Yo Jong, one of her brother’s top foreign policy officials, called the activists “defector scum” and issued what appeared to be a threat of retaliation.

“When you do something you were clearly warned not to do, it’s only natural that you will find yourself dealing with something you didn’t have to,” she said, without specifying what the North would do.

After previous leafleting by South Korean activists, North Korea launched more than 1,000 balloons that dropped tons of trash in South Korea, smashing roof tiles and windows and causing other property damage.

Kim Yo Jong previously hinted that balloons could become the North’s standard response to leafleting, saying that the North would respond by “scattering dozens of times more rubbish than is being scattered on us.”

In response, South Korea resumed anti-North Korea propaganda broadcasts with military loudspeakers installed at the border for the first time in years, to which Kim Yo Jong, in another state media statement, warned that Seoul was “creating a prelude to a very dangerous situation.”