Six news barges and vessels were detected gathered around the submarine construction hall which was not previously present, according to the 38 North, a programme of nonpartisan think tank Stimson Centre which monitors North Korea.
The images were taken on Sunday, 18 September.
It said the new development suggests that “North Koreans are preparing to launch a new submarine” from the shipyard - where Kim Jong-un visited in 2019.
“While barges and a dry dock have been occasionally observed around the submarine launch quay at the main construction hall, the presence of six vessels and barges in this area has not been observed before,” the report said.
However, these new barges were not present in the 12 September imagery taken by another think tank Center for Strategic and International Studies.
The CSIS’ Beyond Parallel report found new construction in maintainance hall and in the secure boat basin area, but said “no significant activity is observed indicating imminent preparations” for anticipated launch.
“However, North Korea maintains the capability to undertake any of these actions on short notice should Kim Jong-un decide to do so,” it added.
The construction of a new ballistic missile submarine (SSB) began in 2016 and in 2019 state media showed Mr Kim inspecting a previously unreported submarine that was built under “his special attention”. Analysts said his visit could be an indication that the construction was nearing completion.
The reports came as US president Joe Biden accused North Korea of “blatantly violating UN sanctions” in a speech to the UN General Assembly on Wednesday as he cited the country’s missile development programme among the reasons to strengthen international nuclear nonproliferation efforts.
The US and North Korean officials also traded accusations after a US intelligence report suggested that Russia Russia was in the process of purchasing arms from North Korea, including millions of artillery shells and rockets
The report follows North Korea’s missile testing spree this year, which includes a suspected launch of a submarine-launched ballistic missile in May.
The country has fired more than 30 ballistic missiles this year, marking the largest number of missile launches in a year, adhering to Mr Kim’s vow to make its military “invincible” to counter what he called persistent hostility from the US.