North Korean leader Kim Jong-un has ordered his country to be ready to use its nuclear weapons at any time and the military to be in "pre-emptive attack" mode, state media said.
The comments, carried by the North's official KCNA news agency on Friday, marked a further escalation of tension on the Korean peninsula after the UN Security Council imposed harsh new sanctions against the isolated state on Wednesday for its nuclear programme.
North Korea, known for belligerent rhetoric, has previously threatened pre-emptive attacks on its enemies, including South Korea and the United States.
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," KCNA quoted him as saying.
"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."
Kim criticised South Korean President Park Geun-hye in his first direct published mention of her by name for acting "in league with the US scoundrels," adding, "her hysteria will precipitate only her ruin in the long run," KCNA said.
Kim made the comments as he supervised military exercises involving newly developed rocket launchers, KCNA reported. It did not mention the date of the drills but said the new weapons had South Korea within range.
Military experts, however, doubt North Korea has developed the capability to fire a long-range missile with a miniaturised nuclear warhead.
"There's a difference between having a nuclear test and having a functioning, reliable nuclear weapon you would actually use in conflict that you have 100 percent confidence in," Jim Walsh, a research associate at MIT's Security Studies Programme, told Al Jazeera.
"In the wake of sanctions, it is not surprising that we have harsh language. They [North Korea] are saying they are ready for nuclear action. I don't think it adds up to much...this reaction is sort of par for the course."
South Korea's defence ministry said on Thursday that North Korea launched several projectiles off its coast into the sea up to 150km away, an apparent response to the UN sanctions.
A spokesman for South Korea's Unification Ministry, which handles relations with the North, said Kim's comments were not helpful and may have been intended for the domestic audience to boost morale in the face of the new UN sanctions.
A US Defense Department spokesman, Commander Bill Urban, said, responding to the report: "We urge North Korea to refrain from provocative actions that aggravate tensions and instead focus on fulfilling its international obligations and commitments."
The latest UN sanctions, drafted by the US and China, the North's main ally, punish the isolated country following its fourth nuclear test, in January, as well as last month's satellite launch, which the US and others say was really a test of ballistic missile technology.
Source: Al Jazeera and agencies