North Korea has conducted another ballistic missile engine test, US defence officials told CNN.
The officials on Monday said the latest engine trial is the third such test using similar technology.
The initial assessment indicates that the engine technology could possibly be used in an eventual intercontinental ballistic missile (ICBM).
However, it was not clear whether the engine would require some adjustment to be used in an ICBM, if it can indeed be used that way at all.
Possession of an ICBM could allow North Korea to threaten the continental US, the officials said.
Meanwhile, Washington announced on Monday that the US Marines have deployed F-35Bs to South Korea for the first time as part of an exercise that began on March 24.
“This is the first time we have operated the F-35B in South Korea,” CNN quoted Pentagon spokesman US Navy Captain Jeff Davis as saying.
The new short takeoff and vertical landing capable stealth strike fighter was only first declared operational in July of 2015.
The deployment is part of a training exercise with the South Korean military, an exercise Davis called “defensive in nature”.