Parasites from human faeces and defaced Western clothing were found in the bags of garbage carried by North Korean balloons into the South, Seoul said Monday.

Pyongyang has sent more than a thousand trash-carrying balloons into the South in recent weeks, in retaliation for leaflets sent northwards by activists opposed to North Korean leader Kim Jong Un.

An analysis of the contents of some 70 of the balloons found that they contained soil in which "numerous parasites, such as roundworms, whipworms and threadworms" were detected, South Korea's ministry of unification said in a statement.

This is likely because human faeces was used in the soil instead of chemical fertilizers, it added.

It said there was "no risk of land pollution (or) infectious diseases" from the balloons, as the volume of soil sent was relatively low.

The garbage sent by North Korea also revealed the country's woeful economic state, the ministry claimed, pointing to tattered children's clothes and other garments worn to the point of disintegration as evidence.

In addition, it said some clothes from the balloons were previously donated by a South Korean company during an aid drive and were deliberately defaced -- including being slashed with knives.

This appeared to be North Korea "expressing extreme hostility toward the leaflet campaigns and to highlight the adversarial stance against South Korea", a ministry official said.

The two Koreas have been locked in a tit-for-tat "balloon war", with an activist in the South confirming last week that he had floated more balloons carrying propaganda leaflets towards the North.

Immediately after, Kim Yo Jong, leader Kim Jong Un's powerful sister, warned that North Korea is likely to retaliate.