According to the respected 38 North group, commercial satellite imagery of the Sohae satellite launching station indicates Pyongyang has begun taking down a processing building and a rocket-engine test stand that had been used to test liquid-fuel engines for ballistic missiles and space launch vehicles.

Sohae, on the northwest coast of North Korea, has been used to test rockets, with the aim of putting a satellite into orbit.

But rocket engines are easily repurposed for use in missiles and outside observers say nuclear-armed Pyongyang's space program is a fig leaf for weapons tests.

38 North analyst Joseph Bermudez called the move an "important first step" for Kim in fulfilling commitments he made to Trump during their June summit in Singapore.

Since Sohae is "believed to have played an important role in the development of technologies for the North's intercontinental ballistic missile program, these efforts represent a significant confidence-building measure on the part of North Korea," Bermudez said.

A US defense official, however, told AFP that the Pentagon was not closely tracking activities at Sohae in terms of how it relates to the denuclearization of North Korea.

"It's not on the radar, so to speak," the official said.

On Monday, Trump said he was "very happy" with how talks were progressing with North Korea, after observers and the media highlighted an apparent lack of concrete results since the summit with Kim.