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

The suspension of the military agreement with the North, which President Yoon Suk Yeol approved earlier on Tuesday, is 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 tons 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.

Since then, the North has deployed troops and weapons at guard posts near the military border.

About 50 North Koreans were seen from the South on Tuesday building up a fence, stretching a few hundred metres, leading to a guard post located on a border hill, according to a Reuters witness. (Reporting by Hyunsu Yim; Editing by Christopher Cushing and Michael Perry)