US prisons have reported 30,000 cases of coronavirus among inmates and guards, a US university research group reported Wednesday, as a government study said prison guards could be instrumental in the spread of the disease.

Data compiled from official sources by the COVID-19 Behind Bars Project of the UCLA law school showed that out of more than two million people incarcerated nationwide, there were 21,007 confirmed coronavirus cases, and 295 deaths.

Among prison staff there were 8,754 cases and 34 deaths.

The data was complied from federal, state and local prisons across the country that report it.

Infections in prisons across the country have rocketed in recent weeks even as the national daily total appears to have reached a plateau.

Despite the intense hit of coronavirus on the US population, with 1.2 million confirmed cases and more than 70,000 deaths, some areas are still not open about prison infections, and others have done limited testing.

Many of the more than 1,000 facilities or reporting units count zero or one infection.

But in some, there are hundreds: in the state prison in Marion, Ohio, 2,176 prisoners and 175 staff are reported infected, with 12 deaths.

In another Ohio prison, in Pickaway, 1,670 inmates and 101 staff are infected, and there have been 26 deaths.

And in the Trousdale, Tennessee state prison, 1,285 inmates and 50 staff are COVID-19 positive, with one death recorded.

A report on the epidemic in prisons Wednesday from the Centers for Disease Control said a high level of prison staff are infected.

Using data that was taken earlier and is less complete than the UCLA data, the CDC study noted that half of the incarceration facilities reporting at least one COVID-19 case list only prison staff as infected.

"Because staff members move between correctional facilities and their communities daily, they might be an important source of virus introduction into facilities," the study said.