Activists in South Korea on Saturday sent winter socks carried by gas-filled balloons across the border to the impoverished North, where they can easily be exchanged for food.

One pair of socks is thought to fetch about 22 pounds (10 kilos) of corn -- enough to sustain a person for a month in the hungry communist state.

About 800 pairs of socks were launched by four large balloons across the border from the northern city of Paju on Christmas Eve.

They were sent with leaflets containing a "politically innocuous" message, said Seoul-based aid group North Korea Peace, which plans to launch 1,000 pairs of socks every month.

"The people of the world have not forgotten the hardships of our fellow brothers and sisters in North Korea. Please hang on and survive until the day of reunification," the message said.

The group said socks are precious in the impoverished state, which pours money and resources into building up its 1.2-million strong armed forces under its Songun (military-first) policy.

Temperatures fall well below freezing during the North's harsh winters.

On Wednesday, defectors from North Korea launched some 200,000 leaflets across the border calling for an uprising following the death of leader Kim Jong-Il who died on December 17 aged 69.

South Korean activists have regularly sent leaflets over the border lambasting the ruling Kim dynasty or carrying news of the popular uprisings seen in the Middle East and North Africa in the past year.

The North, which tightly controls news from outside, has threatened to fire across the heavily fortified border to stop the launches.

South Korea has made conciliatory gestures to its neighbour following Kim's death, scrapping a plan to display Christmas lights near their shared border and sending sympathies Tuesday to the North Korean people.

But Seoul said that it would not send an official mourning delegation to Pyongyang.