The specialized website 38 North and the Center for Strategic and International Studies used commercial satellite imagery to track construction at the site - which they said began before last week's aborted summit in Hanoi between Trump and North Korea's Kim Jong Un.
South Korea's military said it is carefully monitoring North Korean nuclear and missile facilities after the country's spy agency told lawmakers that new activity was detected at a research center where the North is believed to build long-range missiles targeting the USA mainland.
Satellite images taken weeks after the first Trump-Kim summit a year ago in Singapore had shown the North was taking steps to dismantle the rocket engine test stand and the rail-mounted transfer structure at the launch pad.
Following the stalemate in Hanoi, researchers said this week that Pyongyang had started rebuilding the Sohae long-range rocket site after Kim had agreed past year to shut it as part of confidence-building measures.
But he said he would be "very disappointed" if North Korea resumed the testing of nuclear weapons and ballistic missiles.
"I would be surprised in a negative way if he did anything in a negative way that was not per our understanding", he said.
South Korean President Moon Jae-in has replaced his unification minister who played a major role in detente with North Korea over the past year, his office said on Friday, and named a longtime confidant to lead a drive for "a new Korean peninsula".
Asked the same day if he was disappointed about the recent North Korean activity, Trump told reporters, "It's disappointing", while adding without elaborating, "We'll see".
Meanwhile, a USA -based think tank has said it appears the North is rebuilding its ballistic missile test facility, which it had partially dismantled.
He said Trump has made abundantly clear to Chairman Kim that he is personally interested in taking North Korea in this direction if Pyongyang gives up all of its weapons of mass destruction and the means of delivery.
The reports of North Korean activity raise more questions about the future of the dialogue Trump has pursued with Kim after a second summit between them broke down in Vietnam last week. Biegun, the US envoy to North Korea, gave members of the Senate Foreign Relations Committee a classified update Tuesday afternoon on Capitol Hill.
"We have to watch to see what else happens", Wit said. The news follows on the heels of bombshell reports, which said North Korea is rebuilding its main rocket launch site.
Let me read you a statement by a senior State Department official from yesterday: "Nobody in the administration advocates a step-by-step approach".
North Korea has frozen its missile tests, but it would not be surprising to see tensions rise if there is no progress toward a deal.