North Korea on Friday threatened to turn Seoul into a "sea of fire" unless South Korean activists stop the launches of propaganda leaflets across the heavily fortified border.

The warning came hours after South Korean police blocked activists from launching leaflets amid elevated military tensions on the divided peninsula.

"The puppet forces should not forget even a moment that the whole of South Korea might turn into a sea of fire due to the foolhardy leaflet-scattering operations," the North warned in a statement released through its official Korean Central News Agency (KCNA).

The statement, published jointly by front-line army units, said South Korea should not test the patience of North Korean soldiers.

It accused South Korean activists of having employed hit-and-run tactics in border areas to send anti-Pyongyang leaflets into the North more than a dozen times at night in June, July and August.

Such leaflet launches are "an open declaration of a war" against North Korea, the statement said.

About 100 South Korean police officers earlier formed human barriers to block vehicles carrying around 30 activists to the border town of Paju, from where they had planned to launch helium balloons carrying the leaflets into North Korea.

The pamphlets criticised and mocked the North's ruling Kim dynasty and condemned a landmine attack blamed on Pyongyang that maimed two members of a South Korean army border patrol earlier this month.

South Korea vowed the North would pay a "harsh price" for the attack and this week resumed -- after a decade-long break -- the broadcast of propaganda messages into the North using batteries of powerful loudspeakers along the border.

Seoul also announced a series of heavy-weaponry, live-fire military drills with the United States not far from the border.

- 'Declaration of war' -

The North on Friday denied it was behind the mine blasts, with the powerful National Defence Commission (NDC) saying South Korean accusations that its soldiers had sneaked across the border and planted the mines along a known patrol route were "absurd".

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"If our army really needed to achieve a military purpose, we would have used strong firearms, not three mines," the commission said in a statement carried by KCNA.

The mine blasts came as cross-border tensions were already heating up ahead of two-week long South Korea-US wargames that simulate an invasion by North Korea.

On Thursday, the North labelled the annual "Ulchi Freedom" exercise a "declaration of war" and warned of its ability to make retaliatory strikes against Seoul and the White House.

Despite the elevated tensions, the anti-North activists who were turned back from Paju argued that their balloon launch should have been allowed.

"North Korea deserves merciless punishment for planting the mines," Choi Woo-Won, a leading activist, told reporters.

Park Sang-Hak, who heads a group of defectors called Fighters for a Free North Korea, claimed they had managed to launch some 200,000 leaflets earlier in the day in an unpublicised operation east of Paju.

Pyongyang has long condemned the leaflet launches and threatened to shell the launch sites in retaliation.

In October last year, North Korea border guards attempted to shoot down some helium balloons, triggering a brief exchange of heavy machine-gun fire between the two sides.

The incident scuppered a planned resumption of high-level talks.