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"Through this test, it is assessed that (North Korea) has made meaningful progress in engine performance, but further analysis is needed for exact thrust and its possible uses in future," said South Korean Defense Ministry deputy spokesman Lee Jin-woo.
He said the equipment tested "appears to have one main engine with four auxiliary engines connected to it. We believe this was an attempt to develop a new engine."
Pyongyang, for its part, touted the test as a "great leap forward" in its rocket program.
Space program
According to the official North Korean news agency KCNA, the test measured the thrust power in the combustion chamber, the structural safety and reliability of the engine, and the movement of the turbine pipe.
The KCNA statement mentioned only civil, rather than military, purposes for the new engines, saying it would "help consolidate the scientific and technological foundation to match the world-level satellite delivery capability in the field of outer space development."
"An ICBM variant of the Unha could be sufficiently similar to the space launch vehicle in that it would be very likely to succeed, making it a good candidate for a political demonstration even though the Unha would make for a poor missile."
However, in a recent paper published in the journal Korea Observer, weapons experts Markus Schiller and Theodore Postol wrote that the potential for North Korea to repurpose a space rocket to deliver an ICBM "does not mean that North Korea has the ability, or is likely to have the ability, to use this postulated ICBM to materially threaten the United States with a nuclear attack."
"It is unlikely that North Korea now has a nuclear weapon that weighs as little as 1,000 kg (2,200 lb). It is also unlikely that such a first-generation nuclear weapon would be capable of surviving the unavoidable 50 G deceleration during warhead reentry from a range of nearly 10,000 kilometers (6,200 miles)," they wrote.
Elleman said that the engine appeared "too large" for the ICBM prototypes that North Korea has demonstrated during military parades, but added "we think they could shrink it down and fit it onto either of the prototypes that they have paraded to date."
Rocketing forward
Euan Graham, director of international security at Australia's Lowy Institute, said the North Korea's twin civilian and military rocket programs may have "become so sophisticated they're moving on separate tracks."
Display of force
"(The test) was clearly aimed to put pressure on China and to expose the fecklessness of Tillerson's threat to somehow stop the regime," he said.
Graham added that the value of such testing for internal propaganda purposes should also not be ignored.
"Let's not forget the ideological aspect of this," Graham said. "It's important to them to have this ability to demonstrate technological prowess."
CNN's Taehoon Lee contributed reporting from Seoul and Barbara Starr contributed reporting from Washington.