By Agence France-Presse
"A rocket engine was tested today," the US official said on condition of anonymity, without providing any details.
News of the test came one day after US Secretary of State Rex Tillerson urged China, Pyongyang's sole ally, to put more pressure on the North to rein in its atomic weapons and ballistic missile programs.
Calling North Korea the "top security threat" to the United States, Tillerson said China has a "diplomatic responsibility to exert much greater economic and diplomatic pressure on the regime if they want to prevent further escalation in the region."
President Donald Trump has made halting the North Korean nuclear threat his number one foreign policy priority.
"We watch North Korea's actions closely. But we will not comment on intelligence," Navy commander Gary Ross said in a emailed statement, declining to confirm the rocket engine test.
"We call on the DPRK to refrain from provocative, destabilizing actions and rhetoric, and to make the strategic choice to fulfill its international obligations and commitments and return to serious talks."