Satellite imagery shows North Korea has started dismantling its only known underground nuclear test site in line with an announcement the country made last month, a U.S. institute monitoring the North said Monday.

The imagery from May 7 "provided the first definitive evidence that dismantlement of the test site was already well underway," the U.S.-Korea Institute of Johns Hopkins University said on its 38 North website.

The analysis was made public before North Korea holds a ceremony for dismantling the Punggye-ri site between May 23 and May 25, depending on weather conditions, in the presence of international media.

(File photo showing Kim at a facility thought to be related to nuclear development in North Korea. KNS/Kyodo)

North Korea announced last month it will suspend nuclear tests and intercontinental ballistic missile test-launches, and dismantle the Punggye-ri site ahead of a meeting between U.S. President Donald Trump and North Korean leader Kim Jong Un on June 12 in Singapore.

According to 38 North, the imagery shows several key operational support buildings have been razed, and that some of the rails for mining carts -- which had led from the tunnels to their respective spoil piles -- have apparently been removed.

More substantial buildings, including the two largest buildings at the command center, remain intact and no tunnel entrances appear to have been permanently closed yet, probably because North Korea is keeping their dismantlement for a show at the ceremony.

North Korea has carried out all of its six nuclear tests to date at the Punggye-ri site, starting in 2006 and continuing until the most recent and powerful one in September last year.

The closure of the site is seen as a step toward the denuclearization of North Korea. However, Pyongyang has yet to say whether it will give up its nuclear weapons and dismantle related facilities other than Punggye-ri.