North Korea's leader has warned his country could "initiate arbitrary action" and "completely destroy" South Korea if its security was threatened.
Kim Jong-un added that South Korea's "complete collapse cannot be ruled out".
The remarks appear to be Mr Kim's most direct warnings against Seoul and were reported by state media KCNA on Thursday.
Mr Kim revealed plans to expand the country's nuclear and intercontinental ballistic missiles arsenal, ruling out discussions with South Korea and labelling them as the "most hostile enemy".
He added that the country would focus on expanding its nuclear arsenal, and prospects for bettering relations with the US rested entirely on Washington's attitude.
The comments highlight the increasingly different approaches pursued by Seoul and Pyongyang towards each other.
South Korean President Lee Jae Myung responded to Mr Kim's comments on Thursday by saying the value his country needs to espouse is peace.
"A long-established sense of hostility and confrontation cannot be eliminated in an instant. We must build trust through continuous efforts," Mr Lee said.
However, speaking at the country's week-long Ninth Congress of the ruling Workers' Party, Mr Kim vowed to develop stronger intercontinental ballistic missiles.
The country also wants to develop attack systems that use artificial intelligence, unmanned drones, and weapons that can hit enemy satellites, KCNA said.
The Congress is reported to have wrapped up with a military parade in the capital on Wednesday.
The presence of Mr Kim's daughter, known as Ju Ae, will fuel further speculation over whether she is being groomed as his successor.
North Korea has assembled around 50 warheads and possesses enough fissile material to produce up to 40 more, the Stockholm International Peace Research Institute (SIPRI) estimated last year.
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The thinktank also believes that Pyongyang is accelerating the production of further fissile material.
The North's leader has so far not accepted overtures by US President Donald Trump, whom he met with three times during the 79-year-old's first term, but he has left the door open for discussions with the US.
"If the US withdraws its policy of confrontation with North Korea by respecting our country's current status... There is no reason why we cannot get along well with the US," he said, according to KCNA.
Mr Trump plans to travel to China from 31 March to 2 April.
South Korea's spy agency has speculated that the two leaders could meet around that time.
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