SEOUL/WASHINGTON--Two South Korean envoys will travel to the United States on Thursday to meet National Security Adviser H.R. McMaster and other officials to discuss this week's meeting with North Korean leader Kim Jong Un, officials in Seoul and Washington said.

National Security Office head Chung Eui-yong and National Intelligence Service chief Suh Hoon will also speak with other department heads and later possibly meet either U.S. President Donald Trump or Vice President Mike Pence, a South Korean administration official said Thursday on condition of anonymity.

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

The South Korean envoys are expected to brief U.S. 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.

U.S. Defense Secretary Jim Mattis was guarded when asked on Wednesday about the prospect of North Korean engagement.

"Obviously we're cautiously optimistic that there is some forward progress here," 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 programs rose to the highest level in years in 2017. Pyongyang pursues its weapons programs in defiance of U.N. Security Council resolutions, with shrill, bellicose rhetoric coming from both Kim and 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.

Chung and Suh were part of a delegation that traveled to Pyongyang earlier this week in the first encounter between South Korean government officials and Kim Jong Un, where the North Korean leader said he was open to denuclearization 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 denuclearization of North Korea should be the final goal for talks.

A senior U.S. administration official said on Thursday 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 McMaster. Those discussions would be conducted in private among U.S. officials, the source said.

Chung will visit China and Russia after returning from the United States, while Suh Hoon will head to Japan to brief officials.