Leader Kim Jong-Un has ordered North Korea’s nuclear arsenal to be ready for pre-emptive use at any time, in an expected escalation of military rhetoric.

Leader Kim Jong-Un has ordered North Korea's nuclear arsenal to be ready for pre-emptive use at any time, in an expected escalation of military rhetoric following the UN Security Council's adoption of tough new sanctions on Pyongyang.

The North's nuclear warheads must be deployed "on standby so as to be fired at any moment," Kim was quoted as saying by the North's official KCNA news agency on Friday.

He also warned that the situation on the divided Korean peninsula had become so dangerous that the North needed to shift its military strategy to one of "pre-emptive attack".

While the North is known to have a small stockpile of nuclear warheads,  Washington downplayed Kim's threat as posturing.

"We have not seen North Korea test or demonstrate the ability to miniaturize a nuclear weapon and put it on an ICBM (intercontinental ballistic missile)," a US defense official said AFP.

Still, the official added, "our forces are ready to counter-eliminate strikes if necessary."

Also on Friday, North Korea rejected the UN Security Council's tough sanctions as "unfair, illicit and immoral" and vowed to keep building its nuclear arsenal.

"The strengthening of our nuclear deterrent is a legitimate exercise of our right to self-defense, which will continue as long as the hostile US policy is in place," the foreign ministry said in a statement published by state media.