North Korea is developing a larger-class submarine as the country moves to further develop submarine-launched ballistic missile capability by 2020, a US military expert said Wednesday.

North Korea is building a new bigger class of submarine than the existing Sinpo-class, and the new project seems to have been kicked off several years ago, Joseph Bermudez, co-founder and chief analytic officer of AllSource Analysis, said at a briefing organized by 38 North.

His assessment is in line with outlooks from other North Korean observers who speculated that Pyongyang is seeking a bigger sub that can enhance its SLBM capability. The Sinpos are relatively small subs with a displacement of around 2,000 tons. They are known to have vertical ballistic missile launch tubes built into the sail of the submarine.

On Wednesday, North Korea launched an SLBM from the East Sea, the country's third SLBM test-fire this year.

The missile flew some 500 kilometers before landing in waters within Japan's air defense identification zone, demonstrating an improvement from its previous launch tests.

Bermudez said the latest missile appears to have been fired underwater from a submarine, rather than from a submersible test barge, which he said may have been the case for the North's SLBM test last December.

North Korea is displaying a strong will to build an operational SLBM system under a long-term plan and steady progress has been made, he said.

If North Korea achieved that goal, it would complicate ballistic missile defense planning and operations being sought by South Korea and the US, he also warned.

Assuming the current rate of development, North Korea is on track to develop the capability to strike targets in the region, including Japan, from a missile fired from a submarine by 2020, he noted.

Citing 38 North's satellite imagery taken on Monday near a submarine base near the eastern port city of Sinpo, the analyst said North Korea seems to have made preparations for the Wednesday SLBM launch that day.

The satellite image showed a large construction crane placed close to a submarine moored along a submarine dock.  (Yonhap)