South Korean President Moon Jae-in has said North Korea should take bold steps towards denuclearisation, such as abandoning long-range missiles, to extract US concessions amid a prolonged stalemate in their nuclear negotiations.

Mr Moon also said the North would have to seek a compromise in a second summit between North Korean leader Kim Jong-un and US President Donald Trump, which he said could be imminent, in order to resolve the impasse.

Pyongyang and Washington have been struggling to find a breakthrough despite a pledge by Mr Kim at his landmark summit with Mr Trump in Singapore in June to work towards denuclearisation.

Pyongyang has demanded Washington lift sanctions, imposed because of its nuclear and missile tests, and declare an official end to the 1950-53 Korean War.

Those demands were in response to Pyongyang's initial, unilateral steps toward denuclearisation that included dismantling its only known nuclear testing site and a key missile engine facility.

Mr Moon said North Korea should take more concrete steps, such as abandoning its intercontinental and intermediate-range ballistic missiles and dismantling weapons production lines and other nuclear complexes, to secure US concessions such as sanctions relief.

He said reducing US military commitments, such as a withdrawal of troops or strategic assets from the region, would be an unlikely option for Washington.

"The second summit would be a venue where they settle on terms of what specific measures North Korea will take and how those moves will be reciprocated by the United States," Mr Moon told a news conference at the presidential Blue House in Seoul.

"But US forces in South Korea or strategic assets in places like Guam and Japan are not linked with North Korea alone, as they exist for the stability and peace of the entire Northeast Asia," he said.

Mr Kim renewed his resolve to meet Mr Trump again during his meeting with Chinese President Xi Jinping in Beijing this week.

Mr Moon said Mr Kim's latest trip to Beijing signalled that a second summit was imminent.

Another US-North Korea summit would also expedite an unprecedented visit by the North Korean leader to Seoul, planned for last year but delayed amid the nuclear standoff, Mr Moon said.

"I expect we'll hear about high-level talks between North Korea and the United States before long for their second summit," he added.

US Secretary of State Mike Pompeo made several trips to Pyongyang last year.

However, a meeting between Mr Pompeo and senior North Korean official Kim Yong Chol has yet to be rescheduled after an abrupt cancellation in November.

North Korean state news agency KCNA has said Mr Kim and Mr Xi held "in-depth, frank discussions" on the Korean peninsula and nuclear issues during his stay in Beijing, his fourth since March 2018.

Mr Kim told Mr Xi about the difficulties and concerns in improving North Korea-US relations and denuclearisation negotiations, KCNA said.

Mr Xi agreed that North Korea's reasonable concerns must be addressed properly, it said.

China's official Xinhua news agency also said Pyongyang's "legitimate concerns" must be taken seriously in order to achieve a comprehensive resolution on the Korean peninsula.

Xinhua quoted Mr Xi as saying he hoped that North Korea and the United States could meet each other half way and that the international community expected peaceful negotiations.

"The political settlement of the peninsula issue faces a rare historic opportunity," Mr Xi said.