Two South Korean envoys have travelled to the United States to brief officials on this week's meeting with North Korean leader Kim Jong-un, according to officials in Seoul and Washington.

National Security Office head Chung Eui-yong and National Intelligence Service chief Suh Hoon will meet National Security Adviser HR McMaster and other department heads.

They will also possibly meet either US President Donald Trump or Vice President Mike Pence, according to a South Korean government official.

The official said Mr Chung had already spoken to Mr McMaster on the phone shortly after completing this week's visit to economically and diplomatically isolated North Korea.

The South Korean envoys were expected to brief US officials on North Korea's stance on possible future talks with Washington and its apparent willingness to suspend nuclear tests if the security of the North's government is assured.

US Defense Secretary Jim Mattis was guarded when asked yesterday about the prospect of North Korean engagement.

"Obviously we're cautiously optimistic that there is some forward progress here," Mr Mattis told reporters at the Pentagon.

"But we've been optimistic before, so we're going to have to watch actions and see if they match words," he said.

Tensions over North Korea's nuclear and missile programmes rose to the highest level in years in 2017.

Pyongyang pursues its weapons programmes in defiance of UN Security Council resolutions, with hostile rhetoric coming from both Mr Kim and Mr Trump.

North Korea has boasted of its plans to develop a nuclear-tipped missile capable of reaching the mainland United States.

However, fears of all-out war eased last month, coinciding with North Korea's participation in the Winter Olympics in the South.

Mr Chung and Mr Suh were part of a delegation that went to Pyongyang earlier this week in the first encounter between South Korean government officials and Mr Kim, where the North Korean leader said he was open to denuclearisation talks with the United States.

They also agreed the two Koreas would hold their first summit in more than a decade in late April.

South Korean President Moon Jae-in has said sanctions should not be eased for the sake of talks and that nothing less than denuclearisation of North Korea should be the final goal for talks.

China's foreign minister called on the United States and North Korea to have talks as soon as possible and said peace must prevail.

"Although light has already become visible at the end of the tunnel, the road ahead will not be smooth sailing," the minister said.

A senior US administration official said no decision on the possibility of the United States joining a summit of North and South Korea was expected to be made at this week's meeting with Mr McMaster.

Those discussions would be conducted in private among US officials, the source said.