He was murdered in February 2017 when he was smeared with lethal VX agent by two women who thought they were taking part in a reality TV prank show at Kuala Lumpur airport.
"We have a very good relationship together", Trump said.
A February summit between Kim and Trump in Hanoi, Vietnam had been expected to at least lay out a path forward.
There's a new theory surrounding the murder of Kim Jong Un's half-brother in a Malaysian airport.
"Several former USA officials said the half-brother, who had lived outside of North Korea for many years and had no known power base in Pyongyang, was unlikely to be able to provide details of the secretive country's inner workings", the Journal said. He did not elaborate on the letter's contents other than to say it was "very warm", "very nice", and "very personal".
Preventing the CIA from being able to recruit sources like Kim's late half-brother or highly placed North Koreans would deny the agency valuable insights into its leadership and threats to regional and US security.
In an email to Reuters, Jung Pak - a former senior us intelligence official who now works for the Brooking Institute - said that "the president should understand that to keep the nation safe, the CIA needs to be able to do its job gathering and analyzing intelligence that will support the full range of diplomatic, military and economic policies and initiatives".
"There's always ups and downs, and we remain confident that Kim Jong Un and his government will see a path for a brighter future for the North Korean people", she told reporters.More news: Knicks still ready to make push for Kevin Durant signing
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The comments were not helpful but were not seen as an impediment to United States intelligence efforts, both sources said.
Trump claimed Tuesday that his negotiations have been a success.
"In the meantime, he's kept his word", Trump said.
In May, North Korea conducted a "strike drill" for multiple launchers, firing tactical guided weapons in a military drill supervised by Kim. Investigators widely believe the North Korean communist regime is responsible for the killing.
Trump has made history by meeting twice with Kim in a bid to lure North Korea into giving up nuclear weapons.
The launches are a potential embarrassment for Trump, who has long pointed to a lack of North Korean missile and nuclear tests as evidence his talks with Kim are succeeding.
According to the Journal, the person said Kim Jong Nam had traveled to Malaysia in February 2017 to meet his Central Intelligence Agency contact, although that may not have been the sole goal of the trip.
South Korean President Moon Jae-in, who was instrumental in brokering the Singapore summit a year ago, said on Monday talks were underway for a third North-US meeting, according to South Korea's Yonhap news agency.