38 North said a prototype submarine and submersible missile test stand barge appear ready to be put to sea at any time, but it stressed that "there are no activities suggesting" a test was imminent. In this photo, North Korean leader Kim Jong Un guides on the spot the underwater test-fire of strategic submarine ballistic missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on April 24, 2016. But the new images show that significant work is under way at the Sinpo shipyard on North Korea's eastern coast to expand its construction facilities, and there are hints that another missile-carrying submarine may be under construction.
Mr Abe described North Korea's sixth nuclear test and two missile launches that flew over Japan as a national crisis, as he outlined his priorities in the Japanese parliament.
"Commercial satellite imagery of the Sinpo South Shipyard from November 5 indicates that North Korea is on an aggressive schedule to build and deploy its first operational ballistic missile submarine", wrote 38 North.
There were signs the country was continuing to develop submarine launch ballistic missiles, a new ballistic missile submarine, or a combination, 38 North said.
According to the poll, 25.6 percent said they think Tokyo and Washington should give weight to the use of pressure against North Korea and 28.3 percent said they think so to some extent.
In the recent satellite images, the service tower was observed as having been in the same place as before, suggesting that Pyogyang may be conducting ongoing submarine launch ballistic missiles (SLBM) tests.
Last month, The Diplomat magazine quoted a USA government source as saying US military intelligence had detected a new diesel-electric submarine under construction at Sinpo and dubbed it the Sinpo-C.
"Such testing could support the continued development of SLBMs, a new ballistic missile submarine or a combination of both", defense expert Joseph Bermudez said in his analysis.
U.S. President Donald Trump visited China this month seeking its help to rein in North Korea and a senior Chinese diplomat will visit the North from Friday. USA intelligence officials have declined to comment on this.
Earlier this month, satellite images detected ramped-up activity at North Korea's Punggye-ri nuclear site, raising concerns of a possible imminent nuclear test.