North Korea on Saturday threatened South Korea with "indiscriminate" military strikes unless it halts cross-border propaganda broadcasts, and issued fresh nuclear weapons warnings against the United States.

The threats came amid escalating military tensions on the Korean peninsula following a landmine attack South Korea blamed on the North and ahead of a major South Korea-US joint military exercise condemned by Pyongyang.

They also coincided with celebrations in both Koreas to mark the 70th anniversary of the Korean peninsula's 1945 liberation from Japanese colonial rule.

Initially there were hopes the anniversary might be an opportunity for some sort of inter-Korean rapprochement, but instead ties have spiralled downwards to the familiar accompaniment of angry rhetoric and mutual recrimination.

After three landmine blasts maimed two South Korean soldiers on border patrol, Seoul this week resumed high-decibel propaganda broadcasts across the heavily-militarised frontier, using batteries of loudspeakers that had lain silent for more than a decade.

Pyongyang rejected accusations that it was behind the mine incident as "absurd", and its frontline army border command on Saturday demanded the broadcasts be halted immediately.

- 'All-out' military action -

Failure to do so would trigger "an all-out military action of justice to blow up all means for 'anti-North psychological warfare' in all areas along the front," the command said in a statement carried by the North's official KCNA news agency.

The action will involve "indiscriminate strikes which envisage even possible challenge and escalating counteraction," the statement said.

The threat came a day after North Korea said it would turn Seoul into a "sea of fire" if South Korean activists continue the practice of launching anti-Pyongyang leaflets across the border by helium balloon.

The nuclear-armed North regularly ups its bellicose rhetoric before and during the joint military exercises South Korea holds with its US ally every year, but rarely follows through on its threats.

The last direct attack on the South was in 2010 when the North shelled a South Korean border island, killing four people.

Monday sees the start of the two week-long "Ulchi Freedom" drill which involves tens of thousands of South Korean and US troops in a wargame that simulates an invasion by North Korea.

- Nuclear threats -

On Saturday, the North's powerful National Defence Commission threatened the United States with the "strongest military counter-action" should the joint exercise go ahead.

The North Korean army and people "are no longer what they used to be in the past when they had to counter the US nukes with rifles," the commission said in a statement.

It is now an "invincible power equipped with both latest offensive and defensive means ... including nuclear deterrence," it said.

In a special Liberation Day address in Seoul, South Korean President Park Geun-Hye said the recent landmine attack was a serious breach of the armistice that ended the 1950-53 Korean War, and vowed a tough response to any further provocation by the North.

"North Korea must wake up from its daydream that it can maintain its regime through provocations and threats ... these will only lead to isolation and destruction," Park said.

Because the armistice was never replaced by a full peace treaty, the two Koreas technically remain at war.