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.

Images taken on March 6 showed that a rail-mounted structure to transfer rockets to the launching pad appeared to have been completed and "may now be operational."

Cranes have been removed from the pad, while progress also appeared to have been made on rebuilding the support structure for a rocket engine testing stand.

"Given that construction, plus activity at other areas of the site, Sohae (Satellite Launching Station) appears to have returned to normal operational status," 38 North's report said.

The news will compound the White House's frustration over the lack of progress on talks with the North, following the collapse of a second Trump-Kim summit without so much as a joint statement, let alone an agreement on nuclear disarmament.

The official confirmed that Washington would seek from Pyongyang "clarifications on the purposes" of rebuilding the site, adding so far the US has not reached "any specific conclusion about what's happening there."

"We're watching in real time, as you are, the developments at Sohae," he explained, adding: "We don't know why they are taking these steps."

Kim had agreed to shutter Sohae at a summit with the South's President Moon Jae-in in Pyongyang as part of confidence-building measures, and satellite pictures in August suggested workers were dismantling the engine test stand.

Trump equivocated when asked Thursday if he was disappointed about the news. "We'll see," he said. "We'll let you know in about a year."

The president had declared that it was "too early" to tell if a previous report about activity at the site was true, but said he would be "very, very disappointed in Chairman Kim" if the intelligence checked out.

US media had speculated over whether Trump might tighten the thumbscrews on Pyongyang following the Vietnam summit, by ratcheting up an already crippling sanctions regime.

- 'We have sufficient time' -

State Department spokesman Robert Palladino affirmed Washington's commitment to stay engaged with Kim, however, telling journalists on Thursday the administration was ready for "constructive negotiation."

Palladino would not say if Washington had been in contact with Pyongyang over Sohae, situated on North Korea's northwest coast, or the aborted summit.

And despite the apparent setback, the senior official insisted "we still believe this (denuclearization) is all achievable within the president's first term." Unless re-elected, Trump's term will end in January 2021.

"We have sufficient time," they said, without mentioning a deadline for reaching an agreement so the goal could be met.

"Where we really need to see progress and we need to see it soon is meaningful and verifiable steps on denuclearization as quickly as we can," the official added.

"We are mindful that every day the challenge is greater, the threat posed ... is not going away."

Pyongyang used the site in 2012 and 2016 to launch satellites, a maneuver Western experts believe informs its development of inter-continental missiles capable of striking the United States.

CIA director Gina Haspel said in late January that North Korea remains committed to developing long-range missiles despite its denuclearization talks with the US.

- 'Snapback' -

An analysis by two experts at CSIS said the rebuilding of the launch facility amounted to a "snapback" from the moderate dismantlement North Korea performed after Trump's first summit with Kim in Singapore last year.

Joseph Bermudez and Victor Cha said it showed "how quickly North Korea can easily render reversible any steps taken towards scrapping its WMD program with little hesitation."

They called the North Korean actions "an affront to the president's diplomatic strategy" that also showed Pyongyang's "pique" over Trump's refusal to lift sanctions.

They noted that the activity has continued despite Trump's conciliatory words about Kim since the Hanoi summit, and a US decision to cancel annual large-scale exercises with South Korea that the North has objected to.

The exercises -- Key Resolve and Foal Eagle -- were replaced with a shorter exercise that kicked off this week in South Korea to criticism from the North.