North Korea could be preparing to test a submarine-launched ballistic missile(SLBM), according to U.S. experts on the reclusive state.

The observation was made on Beyond Parallel, a website on North Korea by the Washington-based Center for Strategic and International Studies, on Tuesday based on satellite images of the Nampo Navy Shipyard collected on six dates in April. 

The images show that over the previous four weeks, the North has been working on a cylindrical object mounted in the central position on its submersible missile test stand barge. 

The website noted that this is where a missile launch tube would be mounted before conducting a test launch.  

Beyond Parallel said an operational SLBM capability would enhance the survivability of Pyongyang’s nuclear deterrent, citing that the North Korean leader has made clear his intent to perfect long-range ballistic missiles capabilities to reach the United States.  

The site said though all the latest imagery suggests a mounted SLBM launch tube, this cannot be confirmed and must remain speculative until better resolution images become available. 

As other explanations for the spotted activity, the website included repairs, equipment updates, training, strategic deception, or a combination of these.