Los Angeles ambulance workers have been told to stop taking some patients who have extremely low chances of survival to hospital and to limit oxygen use as record Covid cases overwhelm medical resources.
California has become the latest centre of the US coronavirus pandemic, with about 4,000 deaths in the past two weeks, and hospitals are bracing for another surge now the holidays are over.
In Los Angeles, where packed hospitals have been turning away ambulances, medics were told on Monday not to take adult heart attack patients if they could not be resuscitated in the field.
The county's Emergency Medical Services agency also issued a directive for ambulance crews to save oxygen supplies by only administering it to patients with severely low levels.
Local agency head Marianne Gausche-Hill told CBS that medics were "not abandoning resuscitation" in the field, but that taking patients with "very poor outcomes" who cannot be resuscitated could "impact our hospitals."
California was initially praised for its handling of the pandemic in March but it is now reeling.
In the past seven days, it has recorded more than twice as many cases as any other state, placing great strain on resources.
"Many hospitals have reached a point of crisis and are having to make very tough decisions about patient care," said Christina Ghaly, LA County director of health services.
Firefighters trained as paramedics have been temporarily called up to assist hospital staff, while army engineers were drafted in to increase oxygen delivery systems at six hospitals.
"We're likely to experience the worst conditions in January that we've faced in the entire pandemic, and that's hard to imagine," LA County public health director Barbara Ferrer said on Monday.
Positive Covid-19 cases in the nation's most populous county had doubled in just over a month to 800,000, Ms Ferrer said, with more than one in five people testing positive.
"The steepness of this line is frightening and its implications for our healthcare system, our healthcare workers and all the people we care about," she added.
Vaccinations have begun for emergency healthcare workers and the most vulnerable in California, but Governor Gavin Newsom on Monday said the process has "gone too slowly."
Barely a third of the 1.3 million vaccines that were delivered have been administered so far.