SEOUL/WASHINGTON • North Korean leader Kim Jong Un vowed to meet US President Donald Trump again to continue nuclear negotiations, after a two-day summit between the leaders collapsed this week amid disagreement over sanctions relief and conflicting accounts of Pyongyang's demands.

Mr Kim's pledge was released yesterday through North Korea's state-run news agency KCNA in a report offering a more optimistic outlook than the regime's top diplomats gave in a rare news conference hours earlier.

Mr Kim expressed appreciation for Mr Trump's "active efforts towards results", and called the summit talks "productive".

The US President likewise sounded an upbeat note. He tweeted yesterday that the negotiations with Mr Kim were "very substantive", and that "we know what they want and they know what we must have", but he gave no other details about any next steps.

"Relationship very good, let's see what happens!" he wrote.

Earlier, North Korean Foreign Minister Ri Yong Ho had disputed Mr Trump's claim that Mr Kim had demanded a complete removal of economic sanctions - which the US President said led him to break off talks.

Mr Ri told reporters at a hastily arranged media conference that "first stage" steps such as those Pyongyang proposed are inevitable for the process of complete denuclearisation. He added that North Korea's stance "will never be changed", and that "it could be difficult to meet again".

North Korean Vice-Foreign Minister Choe Son Hui told reporters: "Chairman Kim got the feeling that he didn't understand the way Americans calculate. I have a feeling that Chairman Kim may have lost the will to negotiate with Trump."

The North Korean leader in January made a similar threat to shift towards a "new path" if Mr Trump did not lift sanctions, and then proceeded to meet the US President.

On Thursday, Mr Trump ended the summit early, and said that Mr Kim "wanted the sanctions lifted in their entirety, and we couldn't do that".

In exchange, Mr Trump said the North Korean leader had offered to dismantle his nation's main nuclear facility at Yongbyon.

US Secretary of State Mike Pompeo said earlier yesterday that North Korea lacked complete clarity on what it was prepared to offer on dismantling the Yongbyon nuclear facility.

"They were pretty expansive with respect to what they are prepared to do at Yongbyon, but there was still not complete clarity with respect to the full scope of what it is they were prepared to offer," Mr Pompeo told a news conference during a short stop in Manila. He said the United States was "anxious to get back to the table to continue that conversation".

Mr Ri, however, said North Korea had only asked for relief from sanctions enacted in 2016 and 2017. He said that would mean removing sanctions imposed by five of 11 United Nations resolutions against the country.

Ms Choe said that in return for sanctions relief, the North Korean offer included shuttering its plu-tonium and uranium facilities at Yongbyon under the observation of US experts.

Mr Kim has limited options as international sanctions choke North Korea's faltering economy, and securing some measure of economic support from China, its main trade partner and sole major ally, would likely be crucial for the regime.

Beijing yesterday called for North Korean sanctions relief to be discussed at the United Nations Security Council. Both North Korea and the US note that lifting sanctions is an important part of the denuclearisation process, said Chinese Foreign Ministry spokesman Lu Kang.

"They should be considered simultaneously and resolved together, I think this is a common denominator that should be seized," Mr Lu said at a press briefing.

Noting "the positive developments on the peninsula, especially the steps taken by North Korea on denuclearisation", Mr Lu said the UN Security Council should "start discussions on the reversible clauses of the resolutions".

Several analysts said North Korea's demands for sanctions relief included all of the key sanctions, as the US contends, and would have left the Trump administration little leverage to address other hidden enrichment facilities.

"This is basically asking sanctions relaxation on everything. There is not even a value to consider" the North's offer, said Dr Shin Beomchul of the Asan Institute for Policy Studies in Seoul.