South Korea's military operates giant trucks which print and send thousands of leaflets and transmit broadcasts as part of psychological warfare against North Korea, said a report disclosed Wednesday.

North Korea, which tightly controls news from outside, has responded angrily to past propaganda campaigns by the South's military or private groups and threatened to fire across the heavily fortified border to stop such campaigns.

Details of South Korea's military psychological operations (psy-ops) unit emerged in a defence ministry report to Song Young-Sun, a member of parliament's defence committee.

An aide to Song gave the report to AFP. The defence ministry declined comment to AFP, saying information on psychological warfare is confidential.

The South has five-ton trucks equipped with a satellite data receiver and a printer to publish up to 80,000 leaflets a day, and giant helium balloons to carry leaflets into its isolated communist neighbour, the report said.

"The military is known to launch the balloons twice or three times a month, depending on wind direction and weather conditions," the aide to Song told AFP.

The psy-ops unit has practised producing new anti-Pyongyang messages each month in collaboration with US troops in the South and has developed about 1,300 types of leaflets, said the report.

Seoul's military also has a mobile broadcast vehicle and six relay stations which can transmit to the North, it said.

Experts say the regime in the North has tightened its blockade of outside information following the Arab world's uprisings, fearing copycat disturbances.

The North and South agreed in 2004 to halt official cross-border propaganda. But the South resumed "Voice of Freedom" broadcasts after accusing the North of torpedoing a warship in March 2010 with the loss of 46 lives.

The military balloon launches ended in 2000 when ties improved. They were restarted after the North shelled a border island last November and killed four South Koreans.

Private groups of activists and defectors also launch their own balloons carrying leaflets and DVDs criticising the North's authorities and leader Kim Jong-Il.