Two administration officials familiar with the latest intelligence confirm there are indicators of test preparations that could lead to a potential launch in about two weeks.
US satellites have detected new imagery and satellite-based radar emissions indicating North Korea may be testing components and missile control facilities for another ICBM or intermediate launch, officials say.
The US is watching in particular for further testing of North Korean radars and communications that could be used in a launch. The next test launch would be the first since North Korea successfully launched an ICBM on July 4.
Officials also say that North Korea is continuing to test components to launch a missile from a submarine but the US intelligence assessment is that program remains in early stages.
At the same time, a North Korean submarine was spotted in international waters engaging in "unusual activity," two defense officials said.
North Korea's submarine fleet is believed to encompass around 70 subs, though the majority are quite old and likely cannot fire missiles.
When taken together, these developments are concerning because North Korea says it is trying to develop a missile capable of delivering a nuclear warhead to the United States.
Pyongyang has long maintained the ability to legitimately threaten the United States with a nuclear attack is the only way to protect itself against any US-led attempts at regime change.
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 successful deter an enemy from trying to attack it.
The latest intelligence about a potential second ICBM test comes as the second highest ranking US military officer has warned Congress that North Korea's deception techniques to mask their missile launches have grown in sophistication.
"I am reasonably confident in the ability of our intelligence community to monitor the testing but not the deployment of these missile systems. Kim Jong Un and his forces are very good at camouflage concealment and deception" General Paul Selva, vice chairman of the Joint Chiefs of Staff, told the Senate armed services committee on Tuesday.
Selva gave the strongest public indication so far that the US believes the current North Korean ICBM still has limitations, saying that Pyongyang has yet to demonstrate the "capacity to strike the United States with any degree of accuracy or reasonable confidence of success."
Selva said North Korean guidance and control systems for a long range missile still would have to be improved before a missile could actually strike the US.
When asked about the possibility of a preemptive US military strike, Selva said, "I think we have to entertain that potential option. That would be a policy choice by the President of the United States to execute or not execute that option."
But Defense Secretary James Mattis has long warned against letting the North Korean situation get to the point of a US military strike and has strongly and publicly advocated for a diplomatic solution led by the State Department.
Selva, who is deeply involved in the US nuclear weapons and missile defense programs, noted a parallel line of effort is underway to "provide for the defense of the United States with a suitable ballistic missile defense system that can handle the low volume at this point of missiles that he (Kim Jong Un) might be able to deploy that could strike us here across all of US territory, Alaska, Hawaii and the lower 48."
The preparations for a potential new launch come as the US military has observed North Korea carrying out an "unusual level" of submarine activity as well as testing a critical component of a missile that could potentially be launched from a submarine.
Two US defense officials told CNN that that a North Korean Romeo-class submarine is currently engaged in "unusual deployment activity" in the Sea of Japan/East Sea and has been under way for about 48 hours. The US is observing the sub via reconnaissance imagery and the officials said the submarine's patrol had taken it farther that it has ever gone, sailing some 100 kilometers out to sea in international waters. The submarine's activity was different than the typical training activity usually observed closer to shore, according to the officials.
The diesel-electric-powered North Korean sub spotted far from port is about 65 meters long and the US does not assess it capable of venturing very far from its home port.
The activity caused US and South Korean forces to slightly raise their alert level, according to one official.
The US military pays close attention to North Korean submarine activity following the 2010 Cheonan incident where a North Korean sub torpedoed a South Korean Naval vessel.
The deployment comes days after Pyongyang tested a critical component for a missile that could potentially be launched by a submarine The test took place on land at the Sinpo shipyard in North Korea. The current US intelligence assessment is that the missile program aboard submarines remains in the very early stages.
An ejection test in may tested the missile's "cold-launch system," which uses high pressure steam to propel the missile out of the launch canister into the air before the missile's engines ignite, preventing damage to the submarine or submersible barge that would launch the missile. It is the type of technology that allows missiles to be launched underwater from submarines.
, North Korea conducted what experts believed was its first successful submarine missile test, firing a missile called the the KN-11 or Pukguksong-1.
That missile believed to have been modified for use on land. Both the Pukguksong-1 and Pukguksong-2 -- the land-based variant -- were on display during a military parade in April.