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South Korean police have blocked activists from sending propaganda leaflets over the border, after threats of military action from North Korea.

A group of North Korean defectors now living in the South had planned to launch balloons carrying the leaflets near the border town of Paju.

On Friday Pyongyang threatened to open fire on South Korean territory if the launch went ahead.

Police sealed off the intended launch site early on Monday.

The site would remained sealed off until the activists ''give up their plan'', an unnamed police official told AFP news agency.

"It's for security reasons ... The activists have been blocked from entering the site and will not be able to launch balloons from the park," the official was quoted as saying.

No 'empty talk'

Ahead of the event, the South Korean military went into a heightened state of alert. Hundreds of residents near the border were advised to evacuate.

The activist group had planned to scatter hundreds of thousands of leaflets from Imjingak park, near the demilitarised zone between the two Koreas.

Activists have sent leaflets on many past occasions. North Korea condemns the move but specific threats such as the one on Friday are rare.

"The moment a minor movement for the scattering is captured... a merciless military strike by the Western Front will be put into practice without warning," the North Korean military said in a statement carried by KCNA news agency.

"The surrounding area will become targets of direct firing of the KPA [Korean People's Army]," the statement said. "The KPA never makes empty talk."

Organiser Pak Sang-hak spoke out against the decision to block the launch, calling it "ridiculous".

But South Korea's Unification Ministry called for restraint from both sides.

"We urge the North to stop the threats ... and have constantly asked the civic groups to refrain from such [propaganda] acts, in consideration of inter-Korea relations," a ministry spokesman was quoted as saying by AFP news agency.

North and South Korea remain technically at war following the 1950-53 Korean War, which ended in an armistice.

Tensions have remained high since the sinking of a South Korean navy ship in March 2010 which Seoul blamed on Pyongyang, and the shelling of a border island eight months later.

But North Korea has not fired on the Southern mainland since the Korean War.

The border between the two countries is one the most militarised in the world and many worry even the smallest spark could ignite into a situation much more dangerous, says the ����ý's Kevin Kim in Seoul.

Pyongyang's threat of action came a day after South Korean President Lee Myung-bak landed on the border island for the first time since the shelling in November 2010.