Recent satellite images suggest North Korea has completed the external refurbishment of a shipyard dedicated to building and launching a new class of ballistic missile submarines a US think tank said Wednesday.

While it is unlikely that any such vessel would become operational before 2020, the North's efforts to develop a working submarine-launched ballistic missile (SLBM) is clearly "making progress", according to the US-Korea Institute at Johns Hopkins University.

A credible SLBM capability would take North Korea's nuclear strike threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.

Last month, one of the North's experimental GORAE-class submarines carried out an SLBM test in the East Sea (Sea of Japan), launching a missile that travelled around 19 miles (30 kilometres).

South Korea deemed the test a failure as the missile appeared to have exploded, but analysts at the US Korea Institute said it was a success.

"It was probably intended to be limited, focusing upon the submarine's launch systems, missile ignition sequence and initial guidance operations rather than a full operational test," it said, predicting further similar launches this year before a "full-range" flight test.

Satellite pictures dated April 28, five days after the test, showed post-launch maintenance activity being carried out on the submarine at the North's Sinpo South Shipyard.

They also indicated that external work on the yard's submarine construction halls had been completed, and a ramp where new vessels are launched was nearly finished, the institute said.

"When complete, the North will be able to build and launch submarines much larger than the GORAE-class -- including a new class of ballistic missile submarines," it added.

South Korea is particularly concerned by the North's SLBM development, and its defence minister, Han Min-Koo warned Tuesday that Seoul had been slow to respond.

"I don't think there's much time left for us to come up with means to cope with the threat from North Korea's SLBM," Han told a parliamentary committee.