Deadly infections with bacteria that resist even the strongest antibiotics are on the rise in hospitals in the United States, and there is only a “limited window of opportunity” to halt their spread, health officials warned Tuesday.

The bacteria, normally found in the gut, have acquired a lethal trait: they are unscathed by antibiotics, including carbapenems, a group of drugs that are generally considered a last resort. When these resistant germs invade parts of the body where they do not belong, like the bloodstream, lungs or urinary tract, the illness may be untreatable. The death rate from bloodstream infections can reach 50 percent.

Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention, called the organisms “nightmare bacteria” during a telephone news conference, and noted that they could pass their trait for drug resistance — encoded in a scrap of genetic material called a plasmid — along to other bacteria.

Most people who contract these infections already have other serious illnesses that require complicated treatment and lengthy stays in hospitals, nursing homes or long-term care facilities. One bit of good news, Dr. Frieden said, is that the infections do not seem to have spread beyond hospitals into the community at large. But that could easily happen, he warned.