US President Donald Trump will not meet with North Korean leader Kim Jong-un unless he sees some "concrete" action by Pyongyang, the White House has said.

Mr Trump earlier said he is prepared to meet Mr Kim in what would be the first face-to-face encounter between the two countries' leaders.

The meeting could potentially mark a major breakthrough in nuclear tensions with Pyongyang.

"The president will not have the meeting without seeing concrete steps and concrete actions take place by North Korea, so the president will actually be getting something," White House spokeswoman Sarah Sanders said at a news briefing.

Mr Trump and Chinese President Xi Jinping welcomed the prospect of dialogue with North Korea in a telephone call, the White House said this evening.

"The two leaders welcomed the prospect of dialogue between the United States and North Korea, and committed to maintain pressure and sanctions until North Korea takes tangible steps toward complete, verifiable, and irreversible denuclearisation," the White House said

Earlier today, United Nations Secretary-General Antonio Guterres said he is encouraged by the planned meeting between Mr Trump and Mr Kim.

"He commends the leadership and vision of all concerned and reiterates his support for all efforts towards peaceful denuclearisation of the Korean peninsula in accordance with relevant Security Council resolutions," UN spokesman Stephane Dujarric told reporters.

Mr Kim has "committed to denuclearisation" and to suspending nuclear and missile tests, South Korea's National Security Office head Chung Eui-yong told reporters at the White House yesterday after briefing Mr Trump on a meeting South Korean officials held with Mr Kim earlier this week.

"A meeting is being planned," Mr Trump posted on Twitter after speaking to Mr Chung, setting up what would be his biggest foreign policy gamble since taking office in January 2017.

Mr Chung said Mr Trump had agreed to meet by May in response to Mr Kim's invitation.

A senior US official said it could happen "in a matter of a couple of months, with the exact timing and place still to be determined".

South Korean President Moon Jae-in, who led the pursuit of detente with North Korea during his country's hosting of the Winter Olympics last month, said the summit would set a course for denuclearisation on the Korean peninsula, according to a presidential spokesman.

Mr Trump had agreed to meet Mr Kim without any preconditions, another South Korean official said.

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Asian stock markets rose on the news, with Japan's Nikkei up 0.5% and South Korean stocks up more than 1%. The dollar also rose against the safe-haven Japanese yen.

Mr Trump had previously said he was willing to meet Mr Kim under the right circumstances but had indicated the time was not right for such talks.

He mocked US Secretary of State Rex Tillerson in October for "wasting his time" trying to talk to North Korea.

During a visit to Africa, Mr Tillerson said yesterday that, although "talks about talks" might be possible with Pyongyang, denuclearisation negotiations were likely a long way off.

"Kim Jong-un talked about denuclearisation with the South Korean representatives, not just a freeze," Mr Trump said on Twitter last night.

"Also, no missile testing by North Korea during this period of time. Great progress being made but sanctions will remain until an agreement is reached."

A meeting between Mr Kim and Mr Trump, who have exchanged insults that had raised fear of war, would be a major turnaround after a year in which North Korea has carried out a battery of tests aimed at developing a nuclear-tipped missile capable of hitting the US mainland.

Mr Trump's aides have been wary of North Korea's diplomatic overtures because of its history of reneging on international commitments and the failure of efforts on disarmament by previous US administrations.

South Koreans responded positively to the news, with online comments congratulating Mr Moon for laying the groundwork for the Trump-Kim talks.

Some even suggested Mr Moon should receive the Nobel Peace prize, although scepticism over previous failed talks remained.

North and South Korea are technically still at war after their 1950-53 conflict ended in a ceasefire, not a truce.

Britain says it welcomes US-North Korea talks

Britain will keep up pressure on North Korea even as it welcomes progress towards talks between Mr Kim and Mr Trump, a spokesman for British Prime Minister Theresa May has said.

"We have always been clear that we want Kim Jong Un to change path and put the welfare of his people ahead of the illegal pursuit of nuclear weapons," the spokesman said.

"We will continue to work closely with the US, South Korea and the international community to ensure that pressure on North Korea continues and sanctions are strictly enforced until Kim Jong Un matches his words with concrete actions," he said.

Earlier, Russian Foreign Minister Sergei Lavrov hailed the proposed meeting as "a step in the right direction."

He said: "We do hope that this meeting is going to take place. Certainly, it is required to normalise the situation around the Korean peninsula."