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.
Mark Fitzpatrick, International Strategic Studies says, "I Am pretty sure that North Korea would be ready to test a nuclear weapon nearly whenever they make a political decision to do so".
The success of this test launch signifies that we have acquired the greatest possible capability according to new South Korean and American missile guidelines.
The U.S. government source believes the rocket engine could propel an ICBM over 55-hundred kilometers and that the goal of the engine is to target the U.S. The source added that the recent engine tested could be seen as the smallest stage of a three-stage ICBM engine - in other words, the final piece of the puzzle.
"We call on the DPRK to refrain from provocative, destabilizing actions and rhetoric, and to make the strategic choice to fulfill its global obligations and commitments and return to serious talks".
Chinese Foreign Ministry spokesman Geng Shuang said China opposed any action that violated UN Security Council resolutions and called for restraint from all parties.
The US Defense Intelligence Agency warned last month that North Korea was on an "inevitable" path to achieving this.
The United States has tried for years to discourage South Korea from developing longer-range ballistic missiles in keeping with the Missile Technology Control Regime, a voluntary global arms-control pact.
In March after North Korea conducted three rocket engine tests.
But a test on June 21 of a new capability being developed by the United States and Japan to defend against shorter-range missiles failed to hit its target, the U.S. Missile Defense Agency said on Thursday.
They also said that military options for North Korea have recently been updated, and will be presented to President Donald Trump for a decision to act if there is a nuclear test.