He said North Korea should bolster its nuclear force in "quality and quantity"
North Korean leader Kim Jong-Un has ordered his country to be ready to use its nuclear weapons at any time.
According to the official KCNA news agency, he also told the country's military to be on "pre-emptive attack" mode in the face of growing threats from its enemies.
It comes after the United Nations adopted its toughest sanctions to date in response to Pyongyang's fourth nuclear test and rocket launch.
Mr Kim said North Korea should "bolster up (its) nuclear force both in quality and quantity" and stressed "the need to get the nuclear warheads deployed for national defence always on standby so as to be fired any moment".
He added: "Now is the time for us to convert our mode of military counteraction toward the enemies into a pre-emptive attack one in every aspect".
North Korea has previously threatened pre-emptive attacks on its enemies including South Korea, Japan and the US.
The Pentagon has downplayed the risk from North Korea's nuclear arsenal.
"The US government assessment has not changed," a spokesman said.
"We have not seen North Korea test or demonstrate the ability to miniaturise a nuclear weapon and put it on an ICBM (intercontinental ballistic missile)".