"It's so easy to be presidential, but instead of having 10,000 people outside trying to get into this packed arena, we'd have about 200 people standing right there", Mr Trump said, pointing at the crowd directly in front of him.
"The perception that sanctions can bring us on our knees is a pipe dream of the people who are ignorant about us", he added.
North Korea says it will never disarm nuclear weapons if it can't trust Washington.
The number of North Korean defectors to South Korea has fallen since Kim Jong-un came to power seven years ago, a South Korean lawmaker has said.
Such demands - as well as a history of North Korea breaking past promises to dismantle its nuclear program - have raised questions as to whether or not the US and North Korea will actually be able to reach a meaningful agreement.
"Without any trust in the USA, there will be no confidence in our national security and under such circumstances there is no way we will unilaterally disarm ourselves first", North Korean Foreign Minister Ri Yong Ho said Saturday at the United Nations.
'He wrote me attractive letters.
Mr Park said tighter border controls between North Korea and China and higher rates charged by people smugglers were key factors. "And they're great letters".
Bill Cosby to Be Sentenced This Week
He could get probation to 30 years in prison on Monday, when he is scheduled for sentencing for three sexual assault convictions. That was Cosby's second trial for these charges - the jury in the first trial deadlocked, and the judge declared a mistrial.
Trump grumbled that commentators would cast him as "unpresidential" for describing Kim in such glowing terms.
Ri said this lack of confidence in Washington is the reason denuclearization discussions have stalled since the historic Singapore summit in June between President Donald Trump and Chairman Kim Jong Un.
However, US Secretary of State Mike Pompeo told the UN Security Council on Thursday that "enforcement of Security Council sanctions must continue vigorously and without fail until we realise the fully, final, verified denuclearisation".
This combination of two file photos shows U.S. President Donald Trump, left, and North Korean leader Kim Jong Un.
At the UN, North Korea repeated its request to the USA to declare the end of the 1950-1953 Korean war, which ended with a ceasefire but without a peace treaty.
The Security Council has unanimously boosted sanctions since 2006 in a bid to choke off funding for Pyongyang's nuclear and ballistic missile programs.
The Trump administration is preparing for a second summit with Kim to talk about denuclearization.