Photos showing a North Korean missile launched from a submarine were manipulated by state propagandists, and the isolated country may still be years away from developing the technology, a top U.S. military official said Tuesday.

North Korea, heavily sanctioned by the United States and United Nations for its missile and nuclear tests, said on May 9 it had successfully conducted an underwater test-fire of a submarine-launched ballistic missile (SLBM) which, if true, would indicate progress in its pursuit of building missile-equipped submarines.

On Wednesday, the North warned Washington not to challenge its sovereign right to boost military deterrence and boasted of its ability to miniaturize nuclear warheads, a claim it has made before and which has been widely questioned by experts and never verified.