NPR said the images of Sanumdong, one of the facilities Pyongyang has used to produce inter-continental ballistic missiles and space rockets, were taken days before US President Donald Trump and North Korean leader Kim Jong Un met in Hanoi for their high-stakes summit, which ended in failure.
The photos by the firm DigitalGlobe show the presence of cars and trucks at the site on February 22, said NPR -- which has exclusive access to the imagery.
It added that rail cars and cranes can also be seen at a yard.
"When you put all that together, that's really what it looks like when the North Koreans are in the process of building a rocket," Jeffrey Lewis, a researcher at the Middlebury Institute of International Studies at Monterey, was quoted as saying by NPR on Friday.
The Sanumdong analysis comes days after the specialised website 38 North and the Center for Strategic and International Studies said Pyongyang may have resumed operations at its long-range rocket launch site at Sohae, based on their study of satellite imagery from March 6.
The development is likely to further compound Washington's frustration over the lack of progress in its bid to get the North to give up its atomic arsenal, especially after the February 27-28 summit between Trump and Kim collapsed without so much as a joint statement -- let alone an agreement on nuclear disarmament.
According to senior US officials, in the week leading up to the Hanoi summit, the North Koreans had demanded the lifting of effectively all UN Security Council economic sanctions imposed on Pyongyang since March 2016.
In return, Pyongyang offered only to close part of the Yongbyon complex, a sprawling site covering multiple facilities. The North is also believed to have other uranium enrichment plants.
But North Korean foreign minister Ri Yong Ho disputed the US account, saying Pyongyang offered to dismantle all "nuclear production facilities in the Yongbyon area" in exchange for partial sanctions relief.
Trump said Friday that his relationship with Kim "remains good", despite the setback in Hanoi.
US officials have said Washington believes the "final, fully verified denuclearisation" of North Korea is still possible by the end of Trump's first term.
Kim released his first public message since the Hanoi summit on Saturday earlier this week, instructing propaganda officials to conduct "positive information" activities to spur scientific and technological development, according to a Saturday report by North Korean state media outlet KCNA.
'When you put all that together, that's really what it looks like when the North Koreans are in the process of building a rocket,' Jeffrey Lewis, a researcher at the Middlebury Institute of International Studies at Monterey, was quoted as saying by NPR on Friday.