North Korea’s Kim Jong Un said there was no reason to avoid talks with the US if Washington stopped insisting his country give up nuclear weapons, but he would never abandon the nuclear arsenal to end sanctions, state media reported on Monday.

"Personally, I still have fond memories of US President (Donald) Trump," KCNA reported the North Korean leader saying in a speech at the Supreme People's Assembly on Sunday. The two leaders met three times during Trump's first presidency.

Kim's comments come as the new liberal government in Seoul urges Trump to take the lead in reopening dialogue with Kim, six years after all peace talks with Pyongyang collapsed over sanctions and nuclear dismantlement.

"If the United States drops the absurd obsession with denuclearising us and accepts reality, and wants genuine peaceful coexistence, there is no reason for us not to sit down with the United States," Kim was quoted as saying.

It is the first time Kim has mentioned Trump by name since the US president's inauguration in January, said Rachel Minyoung Lee, a North Korea expert with the US-based Stimson Center.

"This is an overture," she said. "It is Kim's invitation to Trump to rethink US policy on denuclearisation, the implication being that if the US drops denuclearisation, he could sit face-to-face with Trump."

Kim's warm words towards Trump were a contrast with his strident assertion that he will never give up nuclear weapons or engage in dialogue with South Korea, which he has designated a main enemy.

It was a matter of survival for North Korea to build nuclear weapons to safeguard its security against grave threats from the United States and South Korea, Kim said, listing a series of regular military drills by the allies that he said had evolved into exercises for nuclear war.

South Korean President Lee Jae Myung said in an interview with Reuters that North Korea was building 15 to 20 nuclear bombs a year and any deal that froze that manufacturing would be a useful step towards eventually dismantling the programme altogether.

"Based on that, we can proceed to medium-term negotiations for nuclear weapons reductions, and in the long run, once mutual trust is restored and North Korea’s regime-security concerns are reduced, we can pursue denuclearisation," he said.

Kim bluntly rejected any phased plan, saying recent overtures from Washington and Seoul for dialogue were disingenuous because their fundamental intent to weaken the North and destroy his regime remained unchanged, and that Lee's phased plan was proof of that.

"The world already knows full well what the United States does after it makes a country give up its nuclear weapons and disarms," Kim said. "We will never give up our nuclear weapons."

'CONDITIONS FOR DIALOGUE'

Kim said sanctions had been "a learning experience" and made his country stronger and more resilient.

North Korea has been under UN sanctions and arms embargoes since its first nuclear test in 2006. But while the sanctions have squeezed funding for military development, Pyongyang has continued to make advances in building nuclear weapons and powerful ballistic missiles.

"The reality is that the previous approach of sanctions and pressure has not solved the problem; it has worsened it," South Korea's Lee told Reuters.

Lee had urged Trump to try and meet Kim when the US president visits South Korea next month for an Asia-Pacific summit, but the Stimson Center's Lee said Kim's remarks seemed aimed at blocking the South's involvement.

"Perhaps he wants to get ahead of the Lee government and dissuade the Trump administration from cooperating with South Korea by reiterating that South Korea is a separate country and, therefore, cannot be a party to the North Korean nuclear issue," she said.

The South Korean president said Pyongyang was refusing to talk to the South and he did not believe North Korea and the United States were having concrete discussions, but he believed the phased approach remained the realistic option.

"Our main task now is to create the conditions for dialogue," Lee said.