About 10 North Korean defectors said Monday they had floated tens of thousands of anti-Pyongyang leaflets as they brushed off the North's latest threats of military attacks.

The defectors announced on a Web site that they launched balloons carrying about 120,000 leaflets at 6 p.m. from South Korea's western island of Gangwha, which is close to North Korea.

Park Sang-hak, who has led a high-profile campaign to send anti-Pyongyang leaflets for years, said separately the leaflets criticized North Korea's second hereditary power transfer.

North Korean leader Kim Jong-un took over the communist country following the December death of his father Kim Jong-il. The late Kim inherited power from his father, the country's founder Kim Il-sung, who died in 1994.

The latest campaign comes as South Korean military and police failed to prevent the defectors from floating the leaflets, which North Korea sees as "an undisguised psychological warfare."

Earlier, police banned civilians and vehicles from accessing the defectors' original launch site of Imjingak near the Demilitarized Zone, citing safety concerns over North Korea's warnings, according to the Gyeonggi Regional Police Agency.

On Friday, North Korea said its army will launch a "merciless military strike" if any move to drop leaflets is detected. South Korea's defense minister reacted swiftly, saying his military is prepared to "completely destroy" the origin of a North Korean attack if it occurs.

In Beijing, U.S. special envoy for North Korea policy Glyn Davies urged North Korea to stop threatening South Korea over leaflets.

"It is grossly disproportionate to have threatened to respond to balloons with bombs," Davies was quoted as telling reporters.

The North's unusually strong threat of an attack is the first since North Korean leader Kim Jong-un took over the country. The North has bristled at any outside criticism of its leader and has made similar verbal threats against the South over leaflets in recent years, although no actual attack has occurred.

Still, the South Korean military has stepped up combat readiness by deploying artillery and tank brigades and combat air patrols by F-15K and KF-16 fighters, according military officials.

"If (the North) launches attacks, (the South Korean) military will strongly and thoroughly retaliate against the origin of the attacks and their supporting forces," Defense Ministry spokesman Kim Min-seok said in a briefing.

Despite the North's threats of retaliation, South Korean activists and defectors frequently send hundreds of thousands of leaflets to the North by balloon.

The two Koreas still remain technically at war, as the 1950-53 Korean War ended in a truce, not a peace treaty. (Yonhap News)