Satellite photos indicate North Korea is building a ballistic missile submarine and may be making preparations to test a submarine-launched missile, according to an analysis of the commercial images by experts at a Washington-based think tank.

The photos of Sinpo South Shipyard taken Monday appear to confirm reports by North Korean state media of a “newly built submarine” inspected by the country’s leader, Kim Jong Un, in July, according to Joseph Bermudez and Victor Cha of Beyond Parallel, a research project funded by the Center for Strategic and International Studies (CSIS).

The photos show support vessels and a crane that, similar to past practice by the regime, could be deployed to tow a barge out to sea for a submarine-launched missile test, the analysts said in a written report.

But the two analysts said it was unclear when a test might be carried out, saying “there is no conclusive evidence at the moment that this is a near-term certainty.”

Despite North Korea media reports claiming that the new submarine's deployment is “near at hand,” the analysts said it is more accurate to describe the ballistic missile submarine threat as “emerging rather than imminent.”

Even after it is built, it would take a year or more to test the submarine and render it fully operational, Bermudez and Cha said.

Still, persistent activity at the site over the past three years signals that North Korea is moving closer to securing a capability that would give the regime an alternative means of delivering nuclear weapons, and one that would be harder for the United States and its allies to track.

“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.

The latest “images suggest North Korea is making real progress in developing a second leg of the nuclear triad, bringing them closer to a survivable nuclear force,” the analysts said.

Building a submarine-based nuclear force also would diminish “prospects for full denuclearization,” the report said, which has been a goal of successive U.S. administrations.

President Donald Trump has held two summits with Kim Jong Un to try to persuade the regime to abandon missile and nuclear arsenal in return for the easing of economic sanctions, but so far North Korea has refused. In recent months, North Korea has conducted numerous short-range ballistic missile tests.

North Korea has warned Washington that if it presses ahead with joint military drills with South Korea, it could scrap nuclear talks and resume nuclear and long-range missile testing. The regime has not conducted a long-range missile test since 2017.

Pyongyang said in 2016 that it tested its first submarine-launched ballistic missile. The solid-fuel missile was designed to be carried on the country’s single Sinpo-class submarine, which only has one missile tube. Military analysts have speculated that North Korea could try to build a submarine with multiple tubes.

Satellite photos of the Sinpo South Shipyard and the surrounding area since 2015 have yet to offer conclusive evidence of the construction of a new ballistic missile submarine. But Bermudez and Cha said Tuesday there is “substantial circumstantial evidence,” including, new infrastructure at the site suited to support such a project, improvements to the shipyard’s construction capacity and movement of parts in and out of the site. The Sinpo South Shipyard is one of the few North Korean shipyards capable of constructing submarines and previous submarines were built there.

During the past year, 15 cranes have been installed all along the dock at the shipyard. At other North Korean submarine bases and facilities, similar cranes were used to suspend netting over submarines to inhibit overhead observation, the analysts wrote.