Washington (CNN) Many pundits and politicians have been asking why the US military isn't doing more to help combat the spread of coronavirus in the United States.

In a New York Times op-ed Sunday, New York Gov. Andrew Cuomo called on President Donald Trump to use the Army Corps of Engineers to add more medical facilities, and late last week, former Vice President Joe Biden said the Department of Defense should be planning to deploy.

But even though some National Guard units have been involved in helping with the logistics and planning of the coronavirus response at the state level, the US military's capacity to provide medical support is limited.

There are 51 military hospitals compared to some 6,000 civilian hospitals nationwide, according to one US defense official, meaning the US military has only about 1% of the nation's hospital capacity. Military hospitals admit many fewer patients per year than do civilian hospitals. While the US military does also have 424 medical clinics, hospital ships such as the USNS Comfort, and the ability to construct field hospitals, those facilities are not ideal for treating persons in isolation care who must be quarantined. Hospital ships, for example, treat their patients in open bays.

While calls for the military to aid in the US response to the pandemic have increased in recent days, defense officials say their first priority is maintaining the health, welfare and "readiness" of the 1.36 million members of the active duty military, as well as the 800,000 members of the National Guard and Reservists in addition to some 730,000 civilian employees in the Department of Defense.

Read More