As per latest reports, "The recent satellite imagery has revealed that North could be preparing for a submarine-based ballistic missile test."
Military experts have suggested that, the North could be preparing for an upgrade to its existing submarine's launch systems Pukguksong-1.
The Pukguksong-1 is an SLBM that was first successfully test-launched in August last year.
The tensions between U.S. and North Korea escalated after Pyongyang tested an intercontinental ballistic missile in July that appears to have the range to hit major US cities. The move was condemned by United States, South Korea and Japan.