A U.S. defence official told CNN that North Korea - which has a fleet of approximately 70 submarines - has been performing ejection tests, which is the first stage needed to launch a ballistic missile out of a sub.
An ejection test is created to see whether a missile can be ejected by high pressure steam out of the launch canister in a submarine and into the air before its engine ignites and takes flight.
That method prevents any potential fire damage or mishaps from the missile's ignition sequence.
North Korea is likely to conduct an additional ballistic missile launch and its sixth nuclear test following its latest launch of an intercontinental ballistic missile (ICBM), South Korea's defense ministry said Monday.
However, he didn't stipulate how the United States plans to deal with Pyongyang and Kim Jong-un's regime. Overseen by the third-generation dictator, the midnight-launch of the missile yet again escalated tensions in the region.More news: Google Tracking In-Store Purchases? Privacy Group Asks FTC To Investigate Google
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The Pentagon spokesman argued that the Pyongyang regime has proved the need for THAAD, calling North Koreans "far better spokespeople" on the issue amid the debate over the necessity of the missile interceptors on the Korean Peninsula.
President Donald Trump told reporters at his second full Cabinet meeting that his administration will be able to take care of North Korea but offered no specifics about what he plans to do.
While the first ICBM, the North fired off on 4 July coinciding the US Independence Day, flew for 39 minutes, the second missile on 28 July stayed in the air for 45 demonstrating a longer range.
The development of SLBMs is one of the traditional components of the so-called "Strategic Triad", a nuclear deterrence strategy involving land, air and sea-based attack capabilities.
Earlier this month, CNN also reported citing multiple military officials that the North had sent a diesel-powered Romeo Class sub on an unprecedented patrol and had been outfitting its Gorae Sub with a possible missile launch demonstration tube.