North Korea's top military body launched a blistering personal attack Friday on South Korean President Park Geun-Hye and vowed to push ahead with the country's nuclear weapons programme.

The harsh tone of the attack, attributed to a spokesman from the National Defence Commission (NDC), echoed the bellicose rhetoric employed by Pyongyang when military tensions soared following the North's nuclear test in February.

Referencing Park by name, rather than using the more neutral "chief executive" moniker, the spokesman warned the president that she was steering the Korean peninsula back into a period of dangerous "confrontation".

The commentary, carried by the North's official KCNA news agency, was largely a response to a speech by Park on Tuesday urging Pyongyang to give up its nuclear ambitions.

The president had also talked up the development of a military deterrent capability that would render the North's nuclear weapons "useless".

A day later, visiting US Defense Secretary Chuck Hagel and his South Korean counterpart Kim Kwan-Jin signed a new strategic plan to counter the growing threat of a North Korean nuclear or chemical weapons attack.

"If Park and her group conspire with outsiders under the pretext of leading (North Korea) to 'change'... and force it to dismantle nuclear weapons, it will be little short of digging their own graves," the NDC spokesman said.

"There will be no bigger fool and poorer imbecile than the one who schemes to side with a nuclear-wielding robber and urge one's own kinsmen to lower a knife first," he added.

South Korea is protected by the US nuclear umbrella and there are currently nearly 30,000 US troops stationed in the country.

The NDC spokesman said North Korea would "invariably advance" its development of nuclear weapons, saying they were a vital deterrent to prevent a US nuclear strike.

Pyongyang has called for a resumption of six-party talks on its nuclear programme, but Washington and Seoul insist that it must first take tangible steps to demonstrate its commitment to denuclearisation.

Analysis of recent satellite images suggests the North has made good on promises to restart a mothballed plutonium reactor and may have doubled its uranium enrichment capacity.

The military tensions in March and April that triggered apocalyptic threats of nuclear war had seemed to ease significantly in recent months.

But a rapprochement signalled by the reopening of a joint industrial zone now appears to have run out of steam.

A scheduled reunion last month for family members separated by the 1950-53 Korean War was cancelled at the last minute by Pyongyang, citing South Korean "hostility".

"The hard-won atmosphere of dialogue and peace is turning into that of confrontation and tension," the NDC spokesman warned Park.

"Park should open her mouth to suit her capacity.

"It is said that a tongue wagged thoughtlessly will become a knife cutting one's own head," he said, adding that the North Korean military stood ready to respond to verbal provocation with a "shower of fire".