US officials have detected "unusual" movement of North Korean submarines, near the city of Sinpo, just days after the second successful Inter-continental Ballistic missile launch.
According to reports, the US military has evidence that suggests the North Koreans carried out an ejection test after the missile launch, which is a test that uses pressure to fire the missile into the air for a quick release instead of using the missile's engine ignition.
That method prevents any potential fire damage or mishaps from the missile's ignition sequence.
Officials say it is concerning because they believe it is the fourth test of the year but third test in the last month.
The sea-based tests come off the back of North Korea's latest long-range missile test last week, that saw the missile fly longer, and farther than ever before.
Shortly after the launch, the US Ambassador to the United Nations implied the US government was "done talking about" North Korea and urged its neighbours, such as China, to act.
The North Koreans celebrated the launch with a banquet dinner for high ranking state officials.
The dinner was also attend by leader Kim Jong-un.
Vice-chairman of the Central Committee and the Central Military Commission of the Workers' Party of Korea, Ri Man Gon said the launch was a "world-startling" event.
"[The successful launch] is giving great joy and courage to the army and people of the DPRK as it hit hard the empire of evils," he said.
"The great success in the second test-fire was another thrilling victory in the history of the nation brought about entirely by the gifted wisdom, strong guts and prominent leadership of the respected Supreme Leader."