New satellite images of a key North Korean shipyard show the regimen has completed work on an experimental ballistic missile submarine.

The images, obtained by Washington-based monitoring group 38 North, show several pictures of North Korea's Sinpo South Shipyard taken from September 21.

In the analysis, 38 North's Joseph S Bermudez Jr notes the ongoing activity which has taken place at the site in recent weeks.

According to Bermudez, netting previously around the SINPO-class (1) experimental ballistic missile submarine (SSBA) seen on August 7 has been removed.

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This indicates "whatever work is being done is now completed".

38 North has detailed the activity taking place at the site in recent weeks. Photo / 2017 Planet Labs, Inc.
38 North has detailed the activity taking place at the site in recent weeks. Photo / 2017 Planet Labs, Inc.

However the analysis notes the submarine and submersible missile test stand barge remain in the same positions they were seen in previously.

The website, part of the US-Korea Institute at Johns Hopkins University's Advanced International Studies, stress this doesn't mean any sort of test is imminent.

"The SINPO-class submarine, submersible test stand barge and nearby test stand, however, appear capable of supporting a test at any time of Pyongyang's choosing," 38 North note.

It also suggests the secretive regimen is in the middle of a ship building program but concede it remains unclear exactly what sort of vessel this could be.

The analysis reveals further ongoing activity at port and facilities at Nopyong-ni on the west side of the peninsula has undergone modernisation.

"This program is continuing and progress is observable in the current imagery. Whether this modernisation program is related to the future development and deployment of a ballistic missile submarine capability is unknown," Bermudez writes.

"Slow pace"

Speaking to news.com.au about the analysis, Dr Peter Layton, a visiting fellow at the Griffith Asia Institute at Griffith University, said he was surprised at how little is going on at the site.

Dr Laton said the rate of building new facilities is actually quite slow.

The movement of cranes and parts indicate ongoing shipbuilding program according to 38 North. Photo / 2017 Planet Labs, Inc.
The movement of cranes and parts indicate ongoing shipbuilding program according to 38 North. Photo / 2017 Planet Labs, Inc.

"Given the astonishing speed of the DPRK (Democratic Republic of Korea) in building and testing new rockets the rather placid scene here is interesting," he said.

"The focus to me then seems very much on building a submarine launched ballistic missile not the submarine which will carry it.

"The DPRK might be happy with the design of the first Sinpo class and happy to build the remaining five of the class later after the missile is proven."

On the other hand, he said the Sinpo is fairly small and short range.

"It may not be a viable platform in being able to be easily found and tracked by US and ROK (Republic of Korea) boats," he said.

"From a military viewpoint, a longer range land-mobile missile would seem a much better alternative - being more survivable and giving the same result of warheads on target."

Dr Layton said these photos suggested there could be more to these photos than meet the eye.

"These photos to me suggest that the DPRK submarine-launched ballistic missile (SLBM) program is more a public relations stunt intended to demonstrate DPRK technical prowess and worry others but not a war fighting weapon," he said.

Satellite imagery of North Korea's Sinpo South Shipyard showed ongoing activity at port and facilities at Nopyong-ni. Photo / 2017 Planet Labs, Inc.
Satellite imagery of North Korea's Sinpo South Shipyard showed ongoing activity at port and facilities at Nopyong-ni. Photo / 2017 Planet Labs, Inc.

Dr Layton also said it didn't appear that the DPRK was building a nuclear sub.

He said there has been talk of the DPRK getting some assistance from external sources as their "missile program is extraordinarily broad and fast moving to be accomplished by a technically backward nation."

However he said these photos suggested that the DPRK is not getting any external support for its submarine building program.

Tremors detected

The analysis comes as a 2.9 magnitude earthquake was recorded near North Korea's nuclear test site Punggye-ri.

It follows a 6.1-magnitude quake last month, which Pyongyang claimed was the result of hydrogen bomb test.

The Korea Meteorological Administration said today's quake was a magnitude 2.7 with a depth of 3km in North Hamgyong Province in North Korea.

North Korean leader Kim jong-un attends what is believed to be the launch of a test missile in September. Photo / AP
North Korean leader Kim jong-un attends what is believed to be the launch of a test missile in September. Photo / AP

The United States Geological Survey (USGS) measured the quake at 2.9 magnitude with a depth of 5km and added it could not conclusively confirm its nature.

A statement on the US Geological Survey website reads: "This event occurred in the area of the previous North Korean Nuclear tests.

"The event has earthquake like characteristics, however, we cannot conclusively confirm at this time the nature (natural or human-made) of the event."

A 3.4 magnitude quake last month near the same location sparked fears there could have been another nuclear test.

Rocket men mystery

Meanwhile speculation has been mounting that North Korea could be planning another missile launch or nuclear test soon with two key officials missing from recent public events.

Ri Man-gon, supervisor of the department for the nuclear and missile development, and Kim Rak-gyom have not been seen in some time with a source telling South Korean newspaper Chosun Ilbo, there could be a good reason for this.

The source told the paper there was "little chance" the men were removed from their positions because they were "praised for [recent] major achievements."

"It's highly likely that they were absent because they'd been given an important assignment," the source said.

Threat "manageable"

White House Chief of Staff John Kelly today said the North Korean nuclear and missile threat remains"manageable".

However he said the isolated nation can't be allowed to develop the ability to strike the US homeland.

His more diplomatic approach is in stark opposition to US President Donald Trump who said his top diplomat was "wasting his time" trying to negotiate with the North.

The president has exchanged threats and personal insults with North Korean leader Kim Jong-un, escalating tensions in recent weeks.

Australian Foreign Minister Julie Bishop comes out from the Neutral Nations Supervisory Commission room at the border village of Panmunjom in Paju, South Korea yesterday. Photo / AP
Australian Foreign Minister Julie Bishop comes out from the Neutral Nations Supervisory Commission room at the border village of Panmunjom in Paju, South Korea yesterday. Photo / AP

Gen Kelly said he hopes diplomacy works before the nation can develop its weapons capabilities further, adding Americans should be concerned about North Korea.

He said the North has developed a "pretty good" intercontinental ballistic missile capability and is developing a nuclear re-entry vehicle, which is needed for a missile to survive re-entry into the Earth's atmosphere, the Associated Press reported.

"I think I speak for the administration, that that state can simply not have the ability to reach the homeland," Gen Kelly said.

Australian Foreign Minister Julie Bishop, who is currently in Seoul for talks with her South Korean counterpart, said she hoped there would be no need for military action.

Bishop, who visited the heavily fortified border between the two Koreas, told Sky News Australia continued to support its ally.

"There is a determination on the part of South Korea to bring this tension to an end and compel North Korea back to the negotiating table," she said.

"Australia has reiterated our support for South Korea, we will stand with them in doing what we can to deter North Korea from further illegal ballistic missile and nuclear weapons tests."