South Korean activists on Monday launched tens of thousands of anti-Pyongyang leaflets into North Korea for the second time in three days, amid heightened military tensions on the divided peninsula.

The conservative activists, including many North Korean defectors, used gas-filled balloons to float 100,000 leaflets criticising North Korean leader Kim Jong-Un over the heavily militarised border.

Some carried slogans reading: "Launch a merciless blow of fire to the nuclear maniac Kim Jong-Un!" or "We want Kim Jong-Un's head!"

The same group, called Fighters for Free North Korea, launched 50,000 leaflets on Saturday to mark the anniversary of the sinking of a South Korean warship in 2010.

Seoul blamed a North Korean submarine for the sinking, although Pyongyang has always denied any involvement.

"We plan to launch 10 million leaflets over the next three months, condemning the North's latest atomic test and urging the North to abandon the nuclear programme," the group said on its website.

The leafleting exercise has long angered Pyongyang, which has often threatened military retaliation against the activists.

In October 2014, North Korean frontier guards attempted to shoot down a set of balloons, sparking a brief exchange of fire across the border.

Tensions on the Korean peninsula have been running particularly high since the South kicked off annual joint army drills with the United States this month.

The North has issued a series of threats in recent weeks, including warnings of nuclear attacks against Seoul and Washington.

Seoul and Washington made the already large-scale joint drills bigger than ever this year in response to the North's nuclear test in January and long-range rocket launch a month later.