Kim Jong Un has threatened to expand North Korea's arsenal of nuclear weapons and develop more sophisticated atomic weapons systems unless the US ends what he calls its "hostility" to his regime.

The North Korean leader ordered officials to develop missiles with multiple warheads, underwater-launched nuclear missiles, spy satellites and nuclear-powered submarines, the Korean Central News Agency (KCNA) said.

Speaking at a party congress on Friday, he said thekey to establishing new relations between his country and the US is "whether the United States withdraws its hostile policy".

In what appears to be an attempt to put pressure on the incoming Biden administration, he said North Korea must further strengthen its military and nuclear capability to guard against the threat of a US invasion.

Mr Kim said he was not advocating a pre-emptive strike and would not use the weapons unless his country was itself threatened with a nuclear attack.

The directive came on the fourth day of his country's first ruling party congress in five years.

He also called the US the "biggest enemy" of North Korea and said Washington's hostile policy toward Pyongyang would not change regardless of who occupies the White House, KCNA said.

Mr Biden, who is to take office later this month, called Mr Kim a "thug" during the presidential election campaign and said the "days of cozying up to dictators are over".

Last year, North Korea called Mr Biden a "rabid dog" that needed to be "beaten to death with a stick."

Mr Biden said in October he would only meet Mr Kim if he decommissioned his nuclear weapons, saying "the Korean peninsula should be a nuclear-free zone".

Nuclear diplomacy between North Korea and the US has largely stalled since the breakdown of a second summit between the Mr Kim and outgoing President Donald Trump in February 2019 in Vietnam.

After the failed Hanoi summit, North Korea carried out several short-range missile and other weapons tests.

The two countries have since been in a face-off over the next steps in their negotiations, with North Korea refusing to disarm in return for a reprieve on its sanctions - dashing hopes of denuclearisation on the Korean peninsula.

Last January, Mr Kim accused the US of dragging its feet in nuclear negotiations and said his country will continue developing nuclear programmes and introduce a "new strategic weapon" in the near future.

In October, the secretive country showed off what was thought to be a new intercontinental ballistic missile on the ruling party's 75th anniversary.