US President Donald Trump on Thursday called off a planned summit with North Korean leader Kim Jong Un, even after North Korea followed through on a pledge to blow up tunnels at its nuclear test site.

Referring to a scheduled June 12 meeting with Kim in Singapore, Trump said in a letter to the North Korean leader: "Sadly, based on the tremendous anger and open hostility displayed in your most recent statement, I feel it would be inappropriate, at this time, to have this long-planned meeting." (See the letter in full below)

Trump called it "a missed opportunity" and said someday he still hoped to meet Kim.

Earlier on Thursday, North Korea repeated a threat to pull out of the unprecedented summit with Trump next month and warned it was prepared for a nuclear showdown with Washington if necessary.

[attach id=650162 size="medium" align="left"]It's over... Donald Trump and Kom Jong Un.[/attach]

In a statement released by North Korean media, Vice Foreign Minister Choe Son Hui had called US Vice President Mike Pence a "political dummy" for comparing North Korea - a "nuclear weapons state" - to Libya, where Muammar Gaddafi gave up his unfinished nuclear development programme, only to be later killed by NATO-backed fighters.

A small group of international media selected by North Korea witnessed the demolition of tunnels at the Punggye-ri site on Thursday, which Pyongyang says is proof of its commitment to end nuclear testing.

The apparent destruction of what North Korea says is its only nuclear test site has been widely welcomed as a positive, if largely symbolic, step toward resolving tension over its weapons. North Korean leader Kim has declared his nuclear force complete, amid speculation the site was obsolete anyway.

A letter from U.S. President Donald Trump to North Korean leader Kim Jong Un canceling their upcoming planned summit in Singapore is seen in this photo released by the White House.