North Korea is continuing to work on a submersible missile test stand barge at its Nampo shipyard, a US think tank said Tuesday, suggesting the work could be part of preparations for a ballistic missile test.

Citing satellite imagery taken Monday, the think tank said the North has positioned a "cylindrical object on the submersible missile test barge at Nampo."

It said the image alone could not confirm an imminent missile launch, but noted the round object could be a launch tube for a submarine launched ballistic missile (SLBM).

"This object could be a launch tube for an SLBM, but this cannot be confirmed yet based on available imagery," said Beyond Parallel, a project of the Washington-based Center for Strategic and International Studies think tank.

It, however, noted North Korean leader Kim Jong-un has expressed his desires to "perfect long-range ballistic missiles."

"An operational SLBM capability would enhance the survivability of their nuclear deterrent," it said.

Pyongyang has maintained a self-imposed moratorium on nuclear and long-range missile testing, but said in 2019 that it is no longer bound by such restrictions.

The country has resume its short-range ballistic missile testing after a year-long hiatus in March. (Yonhap)