The images taken on August 26 and published by the Centre for Strategic and International Studies (CSIS) show vessels and cranes at the Sinpo South Shipyard that is considered to be the main submarine development centre of North Korea, Efe news reported.
The pictures of the shipyard suggest "circumstantial evidence of the construction of a new ballistic missile submarine and preliminary evidence indicates possible preparations for a test", according to CSIS analysis of the imagery.
According to CSIS, the images indicate that "North Korea is making real progress in developing a second leg of the nuclear triad, bringing them closer to a survivable nuclear force and lessening prospects for full denuclearization".
The think tank said the images confirm North Korean media reports and pictures released on July 23 of Kim Jong-un's inspection of a newly built submarine.
"We believe this to be the long-expected follow-on ballistic missile submarine (SSB) to the Korean People's Navy's existing SINPO-class experimental ballistic missile submarine (SSBA)," the think tank said.
North Korea claimed in 2016 that it had successfully tested its first submarine-launched ballistic missile. The Sinpo-class submarine carried only one missile tube.
Sinpo-C is a larger model and can potentially carry at least four ballistic missiles, while its predecessor had the capacity of carrying just one.
CSIS said vessels and a crane in the imagery suggest "possible preparations, based on past practice, to tow the missile test stand barge out to sea for an SLBM test flight".
But, it said, there was no conclusive evidence that the missile was going to be launched soon.
The images and their analysis indicating an imminent "existential threat" come after a series of short-range missile tests by North Korea recently.
The new weapons tests were conducted as talks between the US and North Korea over denuclearization remain stalled.
The stand-off began after the failed Hanoi summit held in February between US President Donald Trump and North Korean leader Kim.
There have been no developments in resuming the talks despite an impromptu meeting between Trump and Kim at the inter-Korean border on June 30.
( With inputs from IANS )