"It's hard to say if it happened immediately after the summit and they just rushed everything - I guess it's possible - but it's more likely that it started just before", 38 North Managing Editor Jenny Town told the Washington Post on Wednesday.
"It's too early to see". I would be very very disappointed in Chairman Kim. Beyond Parallel is "a project sponsored by the defense think tank Center for Strategic and International Studies".
Researchers said that the activity may be meant to "demonstrate resolve in the face of United States rejection" of the North's request for an easing of sanctions in return for some dismantlement of nuclear weapons infrastructure.
On Tuesday, several USA think tanks and South Korea's Yonhap news agency reported that satellite pictures showed works were underway to restore the Sohae Satellite Launching Station at Tongchang-ri, which North Korea pledged to dismantle after the summit in Singapore past year.
An article from 38 North, a website specializing in North Korea studies, cited commercial satellite imagery as indicating that efforts to rebuild some structures at the site started sometime between February 16 and March 2.
The NIS said it couldn't confirm the report on Suh's briefing.
The pictures show a moving structure that had been used to carry launch vehicles to a launch pad on rails has been restored, it said.
The North began work to dismantle a missile engine test stand at Sohae previous year after pledging to do so in a first summit with Trump in June.
"Two support cranes are observed at the building, the walls have been erected and a new roof added".
However, a rocket engine test could be performed there and that would send "a really strong signal" to Washington, Wit said.
Impoverished and isolated North Korea conducted its first successful nuclear test in 2006 followed by a string of increasingly successful ICBM launches.
"If they're not willing to do it...we'll look at (increasing) those sanctions up, in fact", said National Security Advisor John Bolton.
On Monday, Secretary of State Mike Pompeo said that even though there was "no commitment" from the North Koreans yet, he was "hopeful" that "we will be back at it, that I'll have a team in Pyongyang in the next couple weeks continuing to work to find those places where there is shared interest".
The nuclear reactor that is believed to have supplied much of the plutonium for North Korea's nuclear weapons appears to have been shut down for the past three months, the United Nations atomic watchdog said on Monday, without suggesting why.
After repeated failures, North Korea successfully put a satellite into orbit for the first time in 2012 in a launch from the site, which is also known as the Sohae Satellite Launching Station.
At their groundbreaking first summit in Singapore a year ago, Kim and Trump produced a vague statement on the "denuclearisation of the Korean peninsula".
The meeting comes a week after President Donald Trump took up the topic during the second U.S.