South Korean activists on Friday launched helium balloons across the North's border carrying millions of leaflets calling for the overthrow of leader Kim Jong-Il.

A conservative group called National Action for Freedom and Democracy in Korea said it sent up 63 large balloons with some 3.6 million leaflets, which also contained news of the Arab uprisings.

The launch came with tensions running high after the North on Wednesday twice fired shells along the disputed Yellow Sea border and the South responded with warning shots.

The balloons were set off in Yeoncheon, 55 kilometres (35 miles) north of Seoul, with timing devices attached to scatter the bundles in the isolated communist state, the group said.

Experts say the regime has tightened its blockade of outside information following the Arab uprisings, fearing copycat disturbances against the ruling Kim dynasty.

The North has reacted angrily to leaflet launches, threatening to open fire to halt what it calls a smear campaign.