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 cancelled 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."

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 behaviour of the United States," Choe said.

News of the cancelled 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 travelling 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. (CNN)

•South Koreans watch a TV news broadcast on North Korea's dismantling of its Punggye-ri nuclear test site Thursday at Seoul Station in Seoul, South Korea

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