North Korean leader Kim Jong Un called the United States his "biggest enemy" and threatened to expand his nuclear weaponry, asking Washington to put a stop to its hostile policies towards Pyongyang.

Kim said the "key to establishing new relations between (North Korea) and the United States is whether the United States withdraws its hostile policy'' on North Korea, the state-run Korean Central News Agency reported on Saturday.

Kim was speaking at a congress of the ruling Worker's Party in the capital.

The North Korean leaders stressed he would not use his nuclear weapons unless “hostile forces'' intend to use theirs against his country first.

However, he added that the North must strengthen its nuclear capability to fend off any potential US invasion.

He ordered officials to develop a more advanced nuclear weapons system with multiple warheads, underwater-launched nuclear missiles, spy satellites and nuclear-powered submarines.

Kim breaks silence on Biden

In what is North Korea's first reaction to the presidential changeover in the United States, Kim said Washington would not change its policies towards Pyongyang, no matter who sits in the White House.

South Korea's spy agency had told lawmakers in November that Kim was worried about President-elect Joe Biden, who is set to take office on January 20.

Biden has branded Kim a "thug."

He has also denounced the summit diplomacy between the North Korean leader and the outgoing US President Donald Trump.

Biden's advisers have predicted a stricter approach towards Pyongyang. 

A direct Biden-Kim meeting also seems unlikely unless North Korea takes serious measures towards denuclearization.

dvv/sms (AP, Reuters)