Satellite images indicate that North Korea may be constructing a new ballistic missile submarine, according to a new report.

Images of the Sinpo South Shipyard suggest circumstantial evidence of the construction of a new ballistic missile submarine, according to the report on Wednesday from the Center for Strategic and International Studies (CSIS).

The evidence supports claims last month in North Korean state media, which said that North Korean leader Kim Jong Un had inspected a newly built submarine at an undisclosed location.

North Korea is thought to have one Sinpo-class submarine in operation, which launched what is believed to be a KN-11 ballistic missile into the Sea of Japan in 2016.

New satellite images show 15 new cranes at the Sinpo South Shipyard in North Korea

View of the construction halls at Sinpo South Shipyard shows indications of a new sub

Last month, North Korean state media released this image of Kim Jong Un, saying that he 'inspected a newly built submarine' at an undisclosed shipyard

However, the Sinpo-class sub only has one missile launch tube, and experts have speculated that the country would attempt to build a sub with multiple launch tubes.

'The construction and commissioning of a true SSB (ballistic missile submarine) capability would represent a significant advancement of the North Korean ballistic missile and nuclear threat and complicate defense planning in the region, given the difficulties of tracking and/or pre-emptively targeting such capabilities,' the analysts said in the new report.

Authors Joseph Bermudez and Victor Cha write that support vessels and a crane in the satellite imagery suggest preparations for a submarine launched ballistic missile test.

However, they caution that it would likely take a year or more of trials and shakedown cruises to make the new sub truly operational, even if a launch trial were imminent.

The photo released on July 23 shows Kim touring a shipyard near the alleged new sub

An image of the support area shows a level of activity that is typical of what has been observed here in the past

An overview of the shipyard facility from Monday shows the latest activity

Sinpo South Shipyard is one of the few North Korea shipyards capable of building submarines.

The report that 15 new cranes have been installed along the dock at the shipyard.

North Korea has threatened to resume long-range missile and nuclear tests if the U.S. does not scrap annual joint war games with South Korea, which began earlier this month.

The regime has responded with a series of short range missile tests, ratcheting up the pressure with Washington.

In Saturday's latest launch, North Korea said that Kim Jong Un supervised the test-firing of a 'newly developed super-large multiple rocket launcher.'

It appeared to be another demonstration of the North's expanding weapons arsenal apparently aimed at increasing its leverage ahead of a possible resumption of nuclear talks with the United States.