(CNN) Some of the most critical US senior military commanders and nuclear and special operations forces are now operating under extraordinary protection measures to ensure that in the event of a sudden security crisis, including any potential nuclear mission, there will be enough healthy troops and leaders to carry out orders as the coronavirus pandemic grows.

There have only been vague references to many of these measures, but taken together, they present a picture of how much worry and effort is going into ensuring the pandemic stops short of impacting the nation's defense.

General Mark Milley, chairman of the Joint Chiefs of Staff, has publicly voiced concern that the military's readiness must be maintained.

"I think we will have moderate to low levels of readiness impacts," Milley said March 24, if the number of military cases remains relatively low.

In Colorado Springs, at the headquarters of NORAD and the Northern Command, so-called distributed operations are now in place. NORAD monitors US airspace against threats and intrusions, including Russian military aircraft. Northern Command is coordinating the military assistance for the pandemic.

"We are isolating specific command personnel involved in critical mission areas, including homeland defense functions," a US military official at the command told CNN. "To ensure we remain capable of defending the homeland despite the pandemic, our command and control watch teams here in the headquarters split into shifts." Some of the watch teams are now working from Cheyenne Mountain Air Force Station, a Cold War-era bunker inside a nearby mountain.

"Additionally, our personnel are operating in pre-determined physical zones within the building. All command members are closely monitoring their own health as well as the health of their family members. This is mitigating exposure to our personnel and their families and to preserve our capability and capacity to execute our homeland defense and COVID-19 missions," the official said.

The strategy of having critical military personnel now work only in pre-specified zones and work in shifts is being replicated in other critical facilities including the Pentagon. Milley and Defense Secretary Mark Esper have been public for weeks about how much they are limiting contact with their own staffs, and not allowing large meetings inside the Pentagon. The numbers of workers coming to the Pentagon has been cut down by well over half.

But deep concern remains that they avoid a significant outbreak among those left. Pentagon leaders privately are adamant they don't want to have to shutter the doors, something that did not even happen on the day of the Sept. 11 attacks.

And at bases around the country and overseas, it may be impossible to contain an outbreak with so many troops living in local towns. That was one of the reasons Esper put a hold on all military moves for 60 days, affecting some 90,000 troops worldwide.

One Pentagon official, however, told CNN that when a worker at the Pentagon has the virus, it may no longer be possible to trace their contacts and only nearby co-workers may be initially notified someone in their office is ill.

All units involved in nuclear weapons operations are also taking specific precautions. Intercontinental ballistic missile crews are rotating to specifically ensure there is always a "clean team" that can take over if others are sick. Crews of submarines carrying nuclear missiles are isolating for several days before heading out into open ocean to ensure they are all healthy for their lengthy sea patrols. And with a jump in virus cases in Louisiana, there is fresh concern about isolating critical B-52 bomber crews at Barksdale Air Force Base, officials say.

Military officials have refused to discuss detailed arrangements for elite units such as the Navy's SEAL Team Six, the Army's Delta Force or highly specialized Air Force units, all of which can be called into action on a moment's notice for missions ranging from hostage rescue to counter terrorism to secret attacks on high-value targets behind enemy lines. But this week, the Army said its own rapid response units would go to the highest level of health protection known as "Delta," which would severely limit troop movements when they are at home or on their respective bases.

As the pandemic grows, the Pentagon has moved to stop public disclosure of where sick troops are located out of "operational security" concerns. The fear is that too much information could give adversaries an idea of potential military vulnerabilities. Although restrictions had been discussed, on Friday all military public affairs officers were officially notified by the Pentagon that the services could only release information of total number of cases and not the locations of the troops. "Numbers of people in isolation, quarantine, or possibly infected will not be released," according to a Department of Defense memo seen by CNN.