SEOUL (AFP) - North Korea on Friday denied that it was behind a series of mine blasts that maimed two South Korean soldiers on border patrol earlier this month and triggered a spike in cross-border tensions.

The powerful National Defence Commission (NDC) said the accusation that its soldiers sneaked across the border and planted the mines along a known patrol route was "absurd".

"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 the North's official KCNA news agency.

In what was the North's first comment on the incident, the NDC suggested the mines were most likely South Korean devices that had been moved out of their original position by flood waters.

Accusing Seoul of seeking to slander North Korea, the commission challenged the South Korean military to provide video evidence to back up its charges. "If they cannot, they should not speak of provocations by the DPRK," it said, using the official acronym for North Korea.

The mines were tripped by a South Korean border patrol on Aug 4 in the demilitarised zone (DMZ) - a buffer zone stretching 2km on either side of the actual frontier line dividing the two Koreas.

One soldier underwent a double leg amputation, while another lost a single leg.

The UN Command that monitors the ceasefire that ended the 1950-1953 Korean War concluded after a special investigation that the devices were North Korean "wooden box" land mines.

It also determined they had been recently laid, ruling out the possibility that there were old mines that had been moved by shifting soil patterns or flooding.

South Korea said it was convinced of the North's responsibility and warned that Pyongyang would pay a "harsh price" for breaching the ceasefire agreement.

Because the ceasefire was never ratified by a peace treaty, the two Koreas remain technically at war.

The South has already responded by resuming high-decibel propaganda broadcasts across the border, using batteries of loudspeakers that had lain silent for more than a decade.

Officials in the South say restarting the broadcasts is only the "first step" in a series of retaliatory measures.

The mine blasts came as cross-border tensions were already heating up ahead of a two-week long South Korea-US wargame that kicks off Monday and simulates an invasion by North Korea.

The North has labelled the annual "Ulchi Freedom" exercise a "declaration of war" and threatened retaliatory strikes against Seoul and the White House.

The North issued its denial of any role in the mine blasts as both Koreas prepared to commemorate the 70th anniversary on Saturday of the 1945 liberation of the Korean peninsula from Japanese rule.

Earlier this year, there had been hopes that the anniversary might be an opportunity for some sort of rapprochement, but efforts to organise a joint commemoration went nowhere.

Pyongyang refused to consider talks because of Seoul's refusal to cancel its military drills with the United States.