Deaths from the coronavirus are skyrocketing in the United States, reaching levels never before seen, largely fueled by relentless surges in California and Arizona.

As the national death toll nears 400,000, weekly deaths in Maricopa County, Ariz., and in Los Angeles and Fresno Counties in California have reached new highs, according to data compiled by The New York Times.

The virus has been raging for weeks in California — especially in Los Angeles County, where Covid-19 has claimed one life about every eight minutes — although state officials said on Wednesday that they were seeing some encouraging signs.

In Arizona over the past week, state officials have recorded the highest number of new coronavirus cases per capita in the country.

Dr. Marjorie Bessel, chief clinical officer for the Banner Health hospitals in Arizona, warned on Wednesday that unless elected leaders and residents did more to stop the spread of the virus, five large health systems risked becoming overwhelmed with patients. More than two-thirds of the state’s intensive care units were full as of Monday, and the hospitals were preparing for a surge of 25 to 50 percent.

“We hope we do not get there,” Dr. Bessel said, adding, “We’re asking you — we’re imploring you — today to help us avoid that.”

She urged officials to adopt a statewide mask mandate and to ban indoor restaurant dining. Certain cities and counties, such as Maricopa and Pima, have full mask mandates, but Gov. Doug Ducey, a Republican, has resisted calls for a statewide order.

Nationwide, the numbers largely remained grim on Wednesday, though in the Northern Plains, cases this week were at about a quarter of their peak in mid-November, when the region was among the hardest hit in the country. There were at least 3,900 virus deaths in the United States reported on Wednesday, a day after the country hit a daily record of more than 4,400.

Earlier in the pandemic, cities bore the brunt of the virus. But now, although metropolitan areas are still suffering, rural communities are, too. Data compiled by The Times shows that deaths have spiked in less populous places, among them Butler County, Kan.; Sevier County, Tenn., and Etowah County, Ala.

“It’s taken a little while, but this highly contagious virus has now spread not only to the suburbs, but also to rural areas,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University. “In Tennessee, we have some very rural counties, and there’s no county that’s unaffected.”

The geographic diversity of the deaths makes clear that the virus has crushed the health care system in the state, Dr. Schaffner said. On Monday, nearly 90 percent of Tennessee’s intensive care units were full.

Yet it is still common in many rural areas to see people gather in large numbers, often not wearing masks. Some wind up in the hospital.

In regular circumstances, patients in rural areas who require a more sophisticated level of care — as gravely ill Covid-19 patients do — are sent to bigger hospitals in cities. But not all of those transfers are possible now.

“Now we can’t take referrals, because we’re full,” Dr. Schaffner said. “No wonder mortality is going up.”