"We know exactly what the risks are - them using negotiations to drag out the length of time they have to continue their nuclear, chemical, biological weapons programmes and ballistic missiles", he said.
Kim asked Xi to help ease the sanctions that have crippled North Korea's economy, and urged China to back Pyongyang in its denuclearisation talks with Washington, the report said.
"There is no evidence that they are decreasing stockpiles, or that they have stopped their production", one official told NBC.
US Secretary of State Mike Pompeo has acknowledged that it could take years to implement any agreement on eliminating North Korea's nuclear stockpile, a hard-won asset that North Korean leaders regard as a guarantor of their survival.
The US President and North Korea's leader have finally met. Trump touted the agreement as a significant step toward peace in the world.
USA intelligence is not certain how many nuclear warheads North Korea has. "We're well aware of what the North Koreans have done in the past".
Appearing on CBS's Face the Nation, Bolton said he did not want to comment on the reports or on "anything related to intelligence" - NBC having quoted more than a dozen senior USA intelligence officials and the Post four.
When grilled by lawmakers about the specifics of the administration's denuclearization plan for North Korea, Pompeo said he was "not prepared to talk about the details of the discussions that are taking place".
However, one official pointed out that, despite the assessment, the talks between the USA and North Korea were positive and Kim's decision to destroy a major missile-testing site and suspend nuclear and missile tests were a step forward.
In a separate interview, Trump told Fox News that he had "a very good chemistry" with Kim at the summit and believes that Kim will carry out his announced intention to end his nuclear weapons development.
TRUMP: I made a deal with him, I shook hands with him.
"We still need to flesh out all the things that underlay the commitments that were made that day in Singapore". I got along really well with Chairman Kim. He spoke with the foreign ministers of China, Japan and South Korea in recent days about the situation with the North, according to the State Department, which has declined to comment on any upcoming travel.
"Now, we'll let their actions speak for themselves", he added.
Pompeo postponed plans to meet with Defense Secretary Jim Mattis and their counterparts from India on July 6, citing unavoidable circumstances, which has fueled speculation he will make a third trip to Pyongyang.
Rapper Nas invested early in PillPack, Amazon's latest buy
On its own, PillPack had raised $118 million in funding from venture capital firms and has been valued at $360 million. In April, CNBC reported Walmart Inc. was in talks to buy the company for "under $1 billion", citing unnamed sources.
Serena Williams seeded 25th for Wimbledon
The 36-year-old American is continuing her return to action after giving birth to her first child, Olympia, in September 2017 . Simona Halep is No. 1 in the women's draw, with defending champion Garbine Muguruza at No. 3 behind Caroline Wozniacki .
Uruguay 1-0 Portugal
Gelson Martins and Raphael Guerreiro are both in contention to start having returned to full training following injuries. The French defeated Argentina 4-3 earlier on Saturday. "Even if he scores three goals , he has a team working for him".
Carlos Vela, Javier Hernandez score in Mexico win
Miguel Layun pressed for a second goal before the break but his shot from a low cross-field drive was tipped away by Jo Hyeon-woo. Son Heung-min scored a consolation goal for South Korea in injury time, sending a powerful shot past goalkeeper Guillermo Ochoa .