South Korea's military on Tuesday said it would resume all military activities along the demarcation line separating the two countries and the North West Islands after suspending an inter-Korean military agreement.

The suspension of the military agreement with the North, which South Korean President Yoon Suk-yeol approved earlier on Tuesday, was in response to North Korea's decision to send hundreds of balloons carrying trash over the border.

"The South Korean military makes it clear that it will take all necessary measures to protect the lives and safety of its people in response to North Korea's provocations," a defence ministry official said in an emergency briefing.

The large-scale spraying of filth balloons has "seriously threatened the safety of our people and caused property damage," the official added.

Pyongyang on Sunday said it had sent up 15 tonnes of wastepaper using 3,500 balloons, while Seoul vowed "unendurable" measures against the North in response, which could include blaring propaganda from loudspeakers directed at the North.

Under the military pact, both countries agreed to "completely cease all hostile acts against each other" that are the source of military tension and conflict, through measures such as the two sides ending military drills near the border.

It was the most substantive deal to come out of months of historic summit meetings between the two Koreas in 2018, but had been all but scrapped when Pyongyang declared last year it was no longer bound by it. (Reuters)