US -based websites recently released satellite photographs indicating that North Korea has restored structures at its long-range rocket launch facility that it dismantled past year.
Also Tuesday, South Korea's Foreign Ministry said it is anxious about the developments at North Korea's rocket launch site, and that Seoul and Washington have been closely monitoring the area.
Bolton said he was "not aware of any" contact with the North since the Hanoi summit but was expecting an update Monday from his South Korean counterpart.
In response to reports of activity around the plant and missile test site, White House National Security Advisor, John Bolton, said that Washington would be "pretty disappointed" if North Korea chooses to go ahead with a ballistic test.
He added, "We see exactly what they're doing now".
Images of Samundong taken on February 22 showed cars and trucks at the site, as well as rail cars and cranes at a yard, U.S. news outlet NPR has reported.
The Sohae facility has been inactive since the first summit last June between Trump and Kim in Singapore.
Staunch U.S. ally Japan is keeping a wary eye on the dialogue between the United States and North Korea amid concern a deal between those old foes could lead to a scaling back of U.S. commitments in East Asia.
Kim, however, has honored an informal freeze on nuclear and missile testing announced last April.
Trump abruptly departed the meeting with Kim after talks broke down without a deal to denuclearize the Korean peninsula.
As North Korea appears to be preparing for a potential missile launch, a report from the Los Angeles Times reports that the North Korean regime is simultaneously lobbying for increased humanitarian aid through the United Nations, for assistance with a food shortage in the country.
North Korea favours a more step-by-approach, with Kim proposing dismantling its Yongbyon nuclear complex in exchange for lifting the main sanctions - a notion Trump refused in Hanoi despite the vaunted "chemistry" between the pair.
The North Korean leader's sibling, Kim Yo-jong, 30, has played an increasingly prominent role in state business over the past few years, including attending her brother's summit meetings with President Trump and other leaders.
Last August, North Korea turned over 55 boxes of war remains; so far the USA has identified three soldiers from those remains and is expecting to announce several more soon.