Punggye-ri, North Korea (CNN) North Korea appeared to destroy at least three nuclear tunnels, observation buildings, a metal foundry and living quarters at its Punggye-ri nuclear test site on Thursday, in a process observed by invited international journalists.

A CNN crew at the remote mountain site in the country's north witnessed explosions at nuclear tunnels 2, 3 and 4, from observation decks about 500 meters away. They were among two dozen journalists invited into the country to observe the apparent destruction of the site.

Hours later, the White House canceled a planned meeting between North Korean leader Kim Jong Un and US President Donald Trump which had been billed as a historic opportunity to bring peace to the Korean Peninsula.

The move followed days of heightened rhetoric from North Korea -- which had itself threatened to pull out. In a letter addressed to Kim, Trump said: "Based on the tremendous anger and open hostility displayed in your most recent statement, I feel it is inappropriate, at this time, to have this long-planned meeting."

He described the cancellation of the summit as being "for the good of both parties, but to the detriment of the world."

A letter from the President to Chairman Kim Jong Un: "It is inappropriate, at this time, to have this long-planned meeting." pic.twitter.com/3dDIp55xu1

In scathing statements earlier Thursday, Choe Son Hui, a vice-minister in the North Korean Foreign Ministry, had called US Vice President Mike Pence a "political dummy" and warned that North Korea's leadership could reconsider the planned summit.

"Whether the US will meet us at a meeting room or encounter us at nuclear-to-nuclear showdown is entirely dependent upon the decision and behavior of the United States," Choe said.

News of the canceled summit reached journalists on the train as they were still on their way back from Punggye-ri to the North Korean coastal city of Wonsan, an 11-hour journey , prompting a sense of shock.

North Korean officials traveling with them did not give an official comment but immediately got up to make phone calls relaying the news to the top, amid an awkward and uncomfortable atmosphere.

Explosives seen in tunnels

North Korea announced on April 20 that the country had "realized nuclear weaponization," and would no longer need to test nuclear weapons. To demonstrate its commitment, it said it would destroy the nuclear test site.

From Will Ripley in North Korea The North Koreans took us to three of the four tunnels on the site. They allowed us to open the tunnel doors, take a look inside; we couldn't actually step into the tunnels. As far as the eye could see, they were rigged with explosives. The North Koreans invited a little more than two dozen of us to witness these explosions. But there were no international experts in this group. There was nobody who could look at the explosions and tell us if they were actually deep enough to destroy the tunnels or if they were simply closing up the entrances. Imagine how the North Koreans are feeling at the moment, given that they just blew up the nuclear site as a sign, they said, of their willingness to denuclearize and commitment to transparency. They were doing this to make a point ahead of the planned summit. That said, the North Koreans had said they were not going to beg for dialogue and they were infuriated by comments made by Vice President Mike Pence and before that by Trump's national security adviser, John Bolton, saying North Korea could follow the "Libya model." Of course, Gadhafi in Libya gave up his nuclear weapons and was overthrown by US-backed rebels and died several years later. They are simply unwilling to accept that here.

North Korea said inviting international media to the event would "ensure transparency of discontinuance of the nuclear test."

The journalists invited to Punggye-ri spent about about nine hours on the ground at the nuclear test site.

No weapons inspectors or nonproliferation experts were invited to witness the event, and it was unclear whether the explosions rendered the tunnels inoperable, or only caused limited damage.

Before the explosions, the journalists said they were invited to look inside three of the four tunnels, which appeared to be rigged with explosives, before moving a safe distance away to witness their detonation.

The amount and type of explosives used were not described by the regime. The CNN journalists present described seeing "soccer ball" sized and shaped explosives, rigged alongside one another, visible for a distance of around 35 meters inside the tunnels.

Officials provided very little technical information and declined to confirm how far the tunnels extended into the mountain.

A May 23 satellite image provided by DigitalGlobe shows the Punggye-ri test site.

A May 24 photo shows an explosion at tunnel 4 of North Korea's nuclear test site.

The journalists watched a succession of explosions, and when they were finished were allowed closer to inspect the damage. Each tunnel was caved in, with rubble blocking the entrance.

From producer Tim Schwarz at Punggye-ri On the journey from Wonsan, the blinds were drawn on all the windows of the train and we were told weren't allowed to look out. We got off the train in Jaedokri, in Kilju Country in northern Hamgyong, and drove to the test site, past a number of small villages of maybe a couple of dozen houses. In all of these villages, there was not a single sign of any person, of any ox or goat or chicken or any animal. The villages were obviously inhabited. It appears that residents were told to keep inside. The fields had been kept neat and tidy. They'd been plowed, there were shoots coming up and they were obviously well-tended. Normally there would be people working but here was no sign of any inhabitants, not a single person. Along the route, there were six or so military guard points with two guards at each one. They were the only people we saw. On the way back, I took a closer look and all the blinds on the houses were drawn.

A deputy director of the North Korean nuclear research institute, who refused to give his name, addressed the journalists at the beginning and the end of the day. "The dismantling of the nuclear test ground conducted with a high level of transparency has clearly attested once again to the proactive and peace-loving efforts of the government of the DPRK," he said after the explosions, referring to North Korea.

North Korea has conducted six nuclear tests at the site, which lies more than 200 miles (370 kilometers) north of Pyongyang, the most recent and powerful of which was in September 2017.

Three tunnels destroyed

There are four tunnels at the site, although journalists only witnessed the destruction of three. A fourth tunnel used for one nuclear test in 2006 had already been shut down, North Korean officials said. The other five tests were all carried out in tunnel 2.

Before Thursday's explosions, experts had warned that the tunnels' destruction could destroy valuable evidence of the state of North Korea's weapons program. They told CNN they would want to take samples, as well as radiation counters to assess the levels in the atmosphere.

Journalists who attempted to take radiation measuring equipment into North Korea said it had been confiscated.

Tom Cheshire, a correspondent with Britain's Sky News, said his team's satellite phone and radiation dosimeter -- a device to measure nuclear radiation -- were taken away by security at Wonsan airport. Chinese journalists also had equipment confiscated.

Photos: North Korea blows up tunnels at nuclear test site Detonation of entrance to tunnel 2 and the observation building next to it. Photos by CNN Beijing bureau chief Tim Schwarz Hide Caption 1 of 13 Photos: North Korea blows up tunnels at nuclear test site The deputy director of the North Korean Nuclear Weapons Institute briefs reporters before the demolitions begin. Hide Caption 2 of 13 Photos: North Korea blows up tunnels at nuclear test site A close-up of a soldier standing guard in front of an observation building at tunnel 2. Hide Caption 3 of 13 Photos: North Korea blows up tunnels at nuclear test site A soldier stands guard in front of the soon to be demolished observation building of tunnel 2. Hide Caption 4 of 13 Photos: North Korea blows up tunnels at nuclear test site Detonation of entrance to tunnel 2 and the observation building next to it. Hide Caption 5 of 13 Photos: North Korea blows up tunnels at nuclear test site Detonation of entrance to tunnel 2 and the observation building next to it. Hide Caption 6 of 13 Photos: North Korea blows up tunnels at nuclear test site Journalists are taken to inspect the aftermath of the explosion at the mouth of tunnel 2. Hide Caption 7 of 13 Photos: North Korea blows up tunnels at nuclear test site Journalists take photographs at the entrance to tunnel 3. Hide Caption 8 of 13 Photos: North Korea blows up tunnels at nuclear test site A soldier stands at the entrance to tunnel 3. Hide Caption 9 of 13 Photos: North Korea blows up tunnels at nuclear test site A view inside tunnel 3. Hide Caption 10 of 13 Photos: North Korea blows up tunnels at nuclear test site The entrance to tunnel 4, an unused tunnel that North Koreans say was ready to conduct advanced nuclear weapons tests. Hide Caption 11 of 13 Photos: North Korea blows up tunnels at nuclear test site A close-up of explosives rigged at the entrance to tunnel 4, an unused tunnel. Hide Caption 12 of 13 Photos: North Korea blows up tunnels at nuclear test site Explosive charges were placed back as far as the eye could see in tunnel 4. Hide Caption 13 of 13

Chang-Hoon Shin, senior research fellow with the Korea Institute for Maritime Strategy, told CNN after the tunnels' destruction that North Korea could have shown itself to be truly transparent by inviting experts to the site -- and warned against assessing the blowing-up of the tunnels as "comparable to the gesture of denuclearization."

Srinivasan Sitaraman, a political science professor at Clark University and expert on North Korea, said Thursday's display did not mean that Pyongyang was giving up its arsenal of nuclear weapons.

"Shutting down this test site suggests that North Korea (maybe) will not test again at this site," he said. "Shutting down of this test site also does not tell us anything about the state of North Korean ballistic missile testing, especially its Hwasong-15 ICBMs," he added, referring to the intercontinental missile North Korea tested for the first time last November

Delicate diplomacy

The White House's decision followed increasing skepticism that Trump and Kim would follow through on plans to meet in Singapore on June 12.

Choe's scathing words about Pence, carried by North Korea's state-run KCNA news agency, came in response to comments the Vice President made about "the Libya model."

The Libya model refers to the deal the US struck with former Libyan leader Moammar Gadhafi to abandon his country's nuclear aspirations. Less than a decade later, he was ousted from power and killed with the help of NATO-backed forces.

Comparisons to Libya were first raised by National Security Adviser John Bolton in late April , and repeated by Pence earlier this week, who also repeated the suggestion that Kim's regime could be threatened if talks fail.

"This will only end like the Libya model ended if Kim Jong Un doesn't make a deal," Pence told Fox News' Martha MacCallum.

North Korea has also taken issue with joint military drills between the US and South Korea, and suddenly postponed inter-Korean talks earlier this month.​