Scientists in the US have warned about the spread of a 'nightmare' drug-resistant bacteria that appears to be flying under the radar.

A family of drug-resistant and potentially deadly bacteria may be spreading more widely and with increasing stealth than previously thought, US researchers have warned.

Researchers at Harvard's T.H. Chan School of Public Health examined antibiotic resistant Enterobacteriaceae (CRE) - a large family of bacteria that includes many of the more familiar pathogens such as Salmonella - found to be causing disease in four US hospitals.

They found a wide variety of CRE species as well as a wide variety of genetic traits enabling the bacteria to resist antibiotics.

The findings of the study, published in the journal PNAS (Proceedings of the National Academy of Sciences) suggest that CRE is more widespread than previously thought, and may well be transmitting from person to person undetected because it shows no symptoms.

This has led to a call from the researchers for greater surveillance of the dangerous bacteria.

"While the typical focus has been on treating sick patients with CRE-related infections, our new findings suggest that CRE is spreading beyond the obvious cases of disease," said William Hanage, associate professor of epidemiology at Harvard Chan School and senior author of the study.

"We need to look harder for this unobserved transmission within our communities and healthcare facilities if we want to stamp it out."

CRE are a class of bacteria resistant to multiple antibiotics, including carbapenems, which are considered last-resort drugs when other antibiotics have failed.

The bacteria tend to spread in hospitals and long-term care facilities, causing thousands of infections and even death each year.

According to the US Centers for Disease Control and Prevention (CDC), the incidence of infections caused by CRE is on the rise.

CDC Director Tom Frieden has called them 'nightmare bacteria' because they are resistant to some of the last-ditch treatments available to doctors battling resistant infections.

The CDC this week said a superbug resistant to every available antibiotic recently killed a woman in Reno, Nevada.

The woman, who had returned from India shortly before falling ill, died in September after 26 different antibiotics failed to cure her infection.