White House counselor Kellyanne Conway on Tuesday played down the significance of the Post report.
"We see them going to work, just as before", a United States official told the Post.
Last week, it appeared North Korea had begun dismantling part of a key rocket launch site, but according to recent reports based on United States intelligence leaks, Pyongyang might still secretly be continuing its nuclear weapons programme.
The Post noted that the new intelligence does not hint at an expansion of North Korea's capabilities but shows that the country is continuing to work on advanced weapons weeks after US President Donald Trump declared on Twitter that Pyongyang was "no longer a nuclear threat". "Just landed - a long trip, but everybody can now feel much safer than the day I took office", Trump tweeted at the time.
Vipin Narang, a North Korea expert and professor of global relations at MIT, noted that this was also clear from the Kim-Trump summit. The trailer appears identical to those used by North Korea in the past to transport ICBMs. Liquid fueling can also take a fairly long time to complete, giving USA spy satellites a period of time to see launch preparations underway. President Obama said that North Korea was our biggest and most risky problem. He added, "sleep well tonight!"
Since late last month, a series of U.S. intelligence reports indicating that the North is increasing the production of enriched uranium, a key component for nuclear weapons, have been leaked to the USA media. "And we're ahead of the game in this way".
More than 7,700 USA troops remain unaccounted for from the Korean War.
Conway also slammed ex-President Obama for never meeting with Kim. Secretary of State Mike Pompeo confirmed these findings last week.
Democrats were markedly less sanguine.
Satellite photos reportedly show that North Korea continues work on developing intercontinental missile technology. He declined to answer questions about whether the North's nuclear program was still advancing and whether Pyongyang was looking to acquire ballistic missiles that could be launched from submarines.
Monday's report, citing intelligence gathered by US spy agencies and unidentified officials, also said there were signs that North Korea is continuing to construct at least one, and possibly two, liquid-fueled intercontinental ballistic missiles (ICBMs) capable of reaching the United States. Mr Trump later said North Korea was "no longer a nuclear threat".
But Pompeo said there was "an very bad long way to go" before North Korea could no longer be viewed as a nuclear threat.
Earlier this month, media reports revealed that the North was secretly operating a suspected uranium enrichment facility, called Kangson.
The general-level officers, including South Korean Major General Kim Do-gyun, were discussing ways to implement April's inter-Korean summit agreements on non-nuclear military issues, but no huge announcement is expected from the talks at the border village of Panmunjom. They recently returned the possible remains of US soldiersfrom the Korean War. In that address, the North Korean leader was unequivocal about what was next, they say.
Pyongyang sees an official end to war as crucial to lowering tension.