Missile experts said it appeared likely the North Koreans had conducted a static test of a rocket engine, rather than a missile launch, which are usually quickly detected by neighboring South Korea and Japan.

“If it is indeed a static engine test for a new solid or liquid fuel missile, it is yet another loud signal that the door for diplomacy is quickly slamming, if it isn’t already,” said Vipin Narang, a nuclear affairs expert at the Massachusetts Institute of Technology in the United States.

“This could be a very credible signal of what might await the world after the New Year.”