A USA 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.
The US has raised concerns about "unprecedented levels" of North Korean submarine activity and underwater launch tests.
CNN said that land-based and submarine-based missiles are considered two-thirds of what is known as the "Strategic Triad", a theory that a state must have land, air and sea-based nuclear attack capabilities to successfully deter an enemy from trying to attack it.
These types of submarine tests are usually done to examine a missile's ability to "cold launch" out of an ejection shoot by high-pressured steam.
But the current USA assessment is that the North's SLBM program remains in the very early stages, it said. The news outlet reported the sub was patrolling the waters off the coast of Japan, some 100 kilometres in global waters.
Citing Korean Central News Agency, the Associated Press reported North Korea leader Kim Jong Un supervised the latest test and expressed "great satisfaction" as a result of the successful launch.
The decision comes three days after North Korea test-fired a second intercontinental ballistic missile, marking a significant step forward in its pursuit of a nuclear-tipped ballistic missile capable of hitting the continental US.More news: Cash Me Outside girl Danielle Bregoli gets five years probation
More news: Trump Wants to Cut Legal Immigration by Half
More news: Abu Dujana not involved in many attacks, was a nuisance: Army
Officials said Friday's ICBM test lasted about 45 minutes and the missile reached an altitude of about 3,700 kilometres before splashing down in the Sea of Japan.
It comes after North Korea fired an intercontinental ballistic missile with a range of 6,500 miles on Friday night - far enough to reach major United States cities.
People in the area at the time report having seen the launch overnight between Saturday and Sunday.
Today's test came amid news Trump is poised to order a military strike against North Korea "within the next year".
"Because of technology, we've become much better at being able to take care of missiles like that, if need be in the air before they get to the ground", Crist said Monday.
"We will handle North Korea", Trump told reporters.