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 US President Donald 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.

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

Pyongyang had used the site in 2012 and 2016 to launch satellites.

Western experts believe the satellite launches inform North Korea's 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' -

Asked Wednesday about the renewed activity at the site, Trump said it was "too early" to tell if an earlier report about activity at the site was true, but if confirmed, he would be "very, very disappointed in Chairman Kim."

Trump and Kim abruptly ended a summit in Hanoi on February 28 without an agreement or even a joint statement.

"Sometimes you have to walk," Trump said at a news conference afterwards.

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.