"The North could be trying to show the U.S. it can always turn back to aggressive posture by rebuilding missile sites in order to gain leverage in future talks, but without actually firing a missile or rocket", said Cheong Seong-Chang, a senior researcher at the private Sejong Institute.
The Chosun Ilbo reports that commercial satellite images have revealed trucks moving what are presumed to be missile components from an assembly facility in the suburbs of Pyongyang to the Tongchang-ri missile test site.
Although Biegun appeared to be in favor of a more incremental approach before the summit, Trump's national security adviser, John Bolton, has long opposed that strategy and has privately expressed his view that Biegun was offering too much to the North Koreans.
"What Kim Jong will ultimately decide to do may very much be his decision and his decision alone", Biegun said, adding that Trump had made clear last week he would be "very disappointed" if North Korea were to resume testing.
Trump, who once warned Pyongyang of "fire and fury" if it threatened the USA, has been trying to negotiate the North's denuclearization with its leader Kim Jong Un, but their recent Hanoi summit ended in failure.
"Well, look, the president has been very clear that he's not going to make the mistakes of prior administrations".
Asked about the images on Wednesday, Trump said "it's too early to see" if North Korea is breaking a promise by rebuilding the site.
February's summit in Hanoi broke up early when the two leaders failed to agree on USA demands for Mr Kim to give up his nuclear weapons and North Korea's call for sanctions to be lifted. And I don't think I will be, but we'll see what happens.
South Korea's Joint Chiefs of Staff (JCS) says Seoul is closely tracking and monitoring North Korea's missile activities in coordination with the United States. "But he's prepared to engage again because he does think that the prospects for North Korea, which he's been trying to persuade Kim Jong-un to accept if they denuclearized, are really quite spectacular", the NSA said.
Instead of a missile test, North Korea might say it was launching a rocket to deliver a satellite into space.
In an interview with ABC, US NSA Bolton said that he will be talking with his South Korean counterpart Monday morning. The summit ended abruptly as the two leaders failed to reach an agreement.
The Yongbyon complex is a critical part of North Korea's nuclear program, but the country is believed to maintain missiles, warheads and enrichment facilities in other areas as well.
With North Korea still under economic sanctions, "the leverage is on our side right now, and not on North Korea's".