(Reuters) - A serious and potentially fatal fungal infection has been observed in at least 13 people in hospitals and nursing homes in the United States since mid-2013, the U.S. Centers for Disease Control and Prevention said on Friday.

The agency said the fungus Candida auris (C. auris) had emerged globally, including in bloodstream infections, in countries that include Britain, India and Israel, and was often resistant to antifungal drugs.

Seven cases between May 2013 and August 2016, that involved patients with serious underlying medical conditions, were reported in New York, Illinois, Maryland and New Jersey. Another six identified later were still being investigated.

Four of the earlier patients died but it was unclear whether their deaths were associated with the fungus, the CDC said.

"We need to act now to better understand, contain and stop the spread of this drug-resistant fungus," CDC Director Tom Frieden said in a statement. "This is an emerging threat and we need to protect vulnerable patients and others."

The agency had issued an alert in June describing the emergence overseas of C. auris. It requested that U.S. laboratories report suspected cases of the fungus and send samples to the agency, as well as state and local healthcare departments.

The statement comes as health authorities around the world also struggle to control bacteria that are resistant to most classes of antibiotics. Known as "superbugs," such bacteria are believed to kill 23,000 Americans annually, and 700,000 people worldwide.

Most fungal strains from U.S. patients showed some drug resistance, although none were resistant to all three antifungal drug classes. By contrast, strains from some other countries have responded to none of the classes.

Researchers believe most U.S. patients were infected locally because none had traveled to or had links to South Asia or South America, where the strains of their C. auris are most common. That suggests the fungus arrived in the United States only in the past few years, the CDC said.

(Reporting by Ransdell Pierson; Editing by Bernadette Baum)