After Bolton referenced the "Libya model" in dealing with North Korea during an April interview, the latter reacted with a strongly worded statement about the former USA ambassador to the United Nations.
In the letter, Mr Trump wrote he felt it was "inappropriate, at this time, to have this long-planned meeting" after "tremendous anger and open hostility" displayed in North Korea's statement calling US Vice President Mike Pence a "political dummy".
The announcement of the timing for the first meeting between the two leaders came just three days after Trump hosted a senior North Korean delegation at the White House bearing a letter from Kim.
According to North Korea's state-run Korean Central News Agency, "a disgusting terrorists' group" directed by CIA and South Korean spies "ideologically corrupted" a North Korean dissident identified as "Kim" and paid the man more than $20,000 to carry out the attack.
The exact venue of the Trump-Kim summit has yet to be confirmed, although Singapore has declared a special event zone including its foreign ministry, the US Embassy, and several large hotels, such as the Shangri-La.
"I can tell you the president has been receiving daily briefings on North Korea from his national security team", said Sanders.
"We're not paying for their expenses", she said.
There was no immediate response from the North Koreans to his comments.
The White House last month unveiled its own commemorative medallion for the summit, showing Trump and Kim looking at each other with their nation's flags behind them. "It's not - I never said it goes in one meeting. I think that has resonated to this whole point now".
In November, at a time when North Korea still clung to its defiant pursuit of nuclear weapons and ballistic missiles, Singapore suspended trade ties with North Korea in line with toughened sanctions imposed by the U.N. Security Council.