North Korea has tested a rocket engine that could be used to launch satellites or intercontinental ballistic missiles (ICBM).
The move follows a similar rocket engine testing by the country last week.
US defence officials believe that the testing is related to North Korea's intercontinental ballistic missile development programme, Reuters reported.
The testing comes amid increasing US concerns about potential missile and nuclear tests by North Korea in the near future.
A US defence official was quoted by CNN as saying that the initial testing showed that the engine could be used in an intercontinental ballistic missile.
However, it is unclear whether the engine must undergo further modifications to be used in an ICBM.
Additionally, the US Marine Corps has deployed F-35B Lightning II multirole fighters to South Korea for the first time.
The US aircraft will take part in a training exercise with the South Korean military, CNN reported citing US Navy captain Jeff Davis.
Davis said: "This is the first time we have operated the F-35B in the Republic of Korea.
Commenting on the deployment, he added: "It goes without saying we are committed to defending the Republic of Korea and Japan against any North Korean aggression.”