U.S. President Donald Trump and North Korea's leader Kim Jong Un shake hands during their summit at the Capella Hotel on Sentosa island in Singapore June 12, 2018.

Satellite images indicate North Korea has begun dismantling key facilities at a site used to develop engines for ballistic missiles, a first step toward fulfilling a pledge made to U.S. President Donald Trump, a Washington-based think tank said on Monday.

The July 20 images showed work at the Sohae Satellite Launching Station to dismantle a building used to assemble space-launch vehicles and a nearby rocket engine test stand used to develop liquid-fuel engines for ballistic missiles and space-launch vehicles, the 38 North think tank said.

"Since these facilities are 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," it said in a report.

Trump said after his unprecedented June 12 summit with North Korean leader Kim Jong Un in Singapore Kim had promised that a major missile engine testing site would be destroyed very soon.

Trump did not identify the site, but a U.S. official subsequently told Reuters that it was Sohae.

An official said on Tuesday South Korea's presidential Blue House was briefed about the site's dismantlement based on government intelligence but did not elaborate.

According to Yonhap, Nam Gwan-pyo, deputy director of the South's national security office, said: "It's better than doing nothing."

"And it seems like they are going step by step toward denuclearization," Nam said.

The 38 North report comes amid growing questions about North Korea's willingness to live up to the commitments Kim made at the June summit, particularly to work towards denuclearization.

U.S. officials have repeatedly said North Korea has committed to giving up a nuclear weapons program that now threatens the United States, but Pyongyang has offered no details as to how it might go about this.

Shares of South Korean companies with exposure to North Korea rose after the news that the satellite site was being dismantled.