North Korean leader Kim Jong-Un says his scientists have successfully miniaturised thermo-nuclear warheads to place on a ballistic missile and create a "true" deterrent, state media said Wednesday.

While Pyongyang has talked of success in mastering miniaturisation before, this is the first time Kim has so explicitly claimed a breakthrough that experts see as a game-changing step for the North's nuclear capabilities.

Kim also stressed that the warheads were "thermo-nuclear" devices, echoing the North's claim that the fourth nuclear test it conducted in January was of a more powerful hydrogen bomb.

"The nuclear warheads have been standardised to be fit for ballistic missiles by miniaturising them," Kim noted during a meeting with nuclear technicians, the North's official KCNA news agency said.

"This can be called a true nuclear deterrent," Kim was quoted as saying.

His comments came a day after the North's powerful National Defence Commission threatened pre-emptive nuclear attacks on South Korea and the US mainland, as Seoul and Washington kicked off large-scale joint military exercises.

Kim echoed the threat, warning that North Korea would "never hesitate to make a pre-emptive nuclear strike" in the event of any effort by the US and its allies to undermine its national sovereignty as a nuclear state.

Military tensions have surged on the divided Korean peninsula since the North's nuclear test in January and a long-range rocket launch last month.

The UN Security Council responded by imposing tough new sanctions last week, which Pyongyang has condemned and labelled as part of a US-led conspiracy to bring down Kim's regime by force.

- 'More powerful' deterrent -

"The stronger our nuclear strike capability gets, the more powerful our deterrent to aggression and nuclear war grows," Kim said.

Experts have been divided on just how far North Korea may have gone in its ability to miniaturise nuclear warheads, although several top US and South Korean military officials have briefed on its likely success.

The issue is key as, while North Korea is known to have a small stockpile of nuclear weapons, its ability to deliver them accurately to a chosen target has been in doubt.

Kim's confirmation still leaves a question mark over the North's ballistic missile capabilities, with many experts believing it is years from developing a working inter-continental ballistic missile (ICBM) that could strike the continental United States.

It is also unclear whether any miniaturised device the North has designed would be robust enough to survive the shock, vibration and temperature change associated with ballistic flight.

North Korea's claim to have successfully tested an H-bomb in January was greeted with scepticism at the time as the estimated yield was seen as far too low for a full-fledged thermo-nuclear device.

However, numerous weapons experts have suggested it may have been a "boosted" fission device, which makes more efficient use of nuclear material and can be made smaller without sacrificing yield.