Photos and infrared imaging, however, indicate vehicles moving in and out of the facility at Sanumdong, but do not show how advanced any missile construction might be.
Based on this and other evidence, US officials reportedly believe that work is underway on at least one and possibly two liquid-fueled ICBMs at the factory.
Asked about the report, the Pentagon declined to comment.
The militaries of North and South Korea have held talks in an attempt to further defuse tensions at the two countries' borders.
Earlier this month, media reports revealed that the North was secretly operating a suspected uranium enrichment facility, called Kangson. Secretary of State Pompeo acknowledged last week in Senate testimony that North Korean factories "continue to produce fissile material" used in making nuclear weapons.
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While North Korea honored its promise to return the remains of fallen USA soldiers during the Korean War, there have been troubling reports that Pyongyang will never truly disarm.
The official did not rule out a one-on-one meeting between Pompeo and his North Korean counterpart on commitments made during the historic Jun 12 summit in Singapore between President Donald Trump and North Korean leader Kim Jong Un.
Sohae has been the main site for North Korea's satellite launches since 2012.
Instead, the Post reported that "senior North Korean officials have discussed their intention to deceive Washington about the number of nuclear warheads and missiles they have, as well as the types and numbers of facilities, and to rebuff global inspectors, according to intelligence gathered by USA agencies".
Their schemes include potentially claiming that they have completely denuclearized by declaring and disposing of 20 warheads while keeping dozens more, the Post wrote. Pompeo said there was "an bad long way to go" before North Korea could no longer be viewed as a nuclear threat.
North Korea also called the talks with a delegation led by Secretary of State Mike Pompeo in July "regrettable", claiming the USA has undermined the spirit of the summit. Many experts say North Korea merely intends to weaken the sanctions and has no intention of fully giving up its nuclear weapons.
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Some of the workers say they were unaware they were entering South Korea until they arrived at the South Korean embassy in Malaysia, the independent National Human Rights Commission of Korea said.
She also said the visit aimed to strengthen Indonesia's "long and historical relationship" with North Korea, forged decades ago by the two nations' respective founding fathers, Sukarno and Kim Il-sung.
"In Singapore I never heard the words: we will unilaterally disarm", Narang said. It had been used to assemble space launch vehicles and develop and test liquid-fuel rocket engines.
The U.S. State Department has said it is committed to building a peace mechanism in place of the armistice when the North denuclearises.
"Meeting with Kim Jong Un was an interesting and very positive experience", Trump said in a tweet.
"It's clear to everyone", he told Pompeo at the July 25 hearing, "that North Korea's dismantling of an outdated missile test facility, as well as a previously dismantled ICBM assembly building - which can be rebuilt within three days - are empty gestures, and not indicative that North Korea has changed its tune".
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The US is deeply suspicious of Iranian activity in the Middle East and is an ally of Israel and Saudi Arabia, two of Iran's foes. In May, Mr Trump withdrew the USA from the 2015 global agreement created to deny Tehran the ability to build nuclear weapons.
The U.S., he added, has "to know what they have before talking about caps and arms control". The commander of US Forces Korea, General Vincent Brooks, said at the recent Aspen Security Forum that North Korea's "production capability is still intact".