CHICAGO >> The world has entered a “new and dangerous phase” of the coronavirus pandemic, a top official from the World Health Organization said today, a stark warning that came as the United States struggled to control spiraling outbreaks and as business leaders signaled growing unease with the country’s ability to effectively contend with the virus.

Coronavirus cases spiked sharply across the American South and West, particularly in states that loosened restrictions on businesses several weeks ago.

In Florida, Oklahoma, South Carolina and Arizona, daily counts of new coronavirus cases reached their highest levels of the pandemic this week. Texas, which has seen known cases double in the past month, became the sixth state to surpass 100,000 cases, according to a New York Times database of U.S. cases.

Around the country, there were indications that major companies and sports teams were changing their own plans as the new surges emerged.

Apple said it was temporarily closing 11 retail stores across four states amid an uptick in cases. AMC Entertainment reversed course on its mask policy today, saying it will now require patrons to wear face coverings when movie theaters reopen next month.

Two Major League Baseball clubs, the Philadelphia Phillies and the Toronto Blue Jays, and a professional hockey team, the Tampa Bay Lightning, abruptly shut down training facilities in Florida over concerns that the virus was threatening players’ safety.

Across the globe, the outlook for containing the coronavirus worsened. A pandemic that had been defined early on by a series of shifting epicenters — including Wuhan, China; Iran; northern Italy; Spain; and New York — was now distinguished by a wide and expanding scope. Eighty-one nations have seen a growth in new cases over the past two weeks. Only 36 have seen declines.

“Many people are understandably fed up with being at home,” Dr. Tedros Adhanom Ghebreyesus, director general of the WHO, said in a news conference in which he described the new phase of the virus. “Countries are understandably eager to open up their societies and their economies. But the virus is still spreading fast. It is still deadly and most people are still susceptible.”

A sobering lesson in the virus’ tenacity came in China, where officials had recently proclaimed that they had vanquished the virus — only to see it surge back in Beijing, the capital. That metropolis, of 21 million people, is facing new restrictions on travel and renewed school closures. Seoul, South Korea, also reported a new surge in cases today.

For weeks, the U.S. has been slowly but determinedly returning to its pre-pandemic existence amid economic turmoil. Businesses reopened, summer camps started and retail workers returned to stores.

But efforts to boost the economy by bringing more people back to work may be happening too soon, experts said, warning that the economic outlook in the U.S. remains wildly uncertain.

Eric Rosengren, the president of the Federal Reserve Bank of Boston and an influential policymaker within the central bank system, cited rising caseloads in South Carolina and Florida as he cautioned of the economic impact of states reopening before the virus was under control. The tension between a tumbling economy and a global pandemic remained stark.

“I expect the economic rebound in the second half of the year to be less than was hoped for at the outset of the pandemic,” Rosengren said, citing the virus’ continuing spread and the acceleration of new cases in many states.

Cases have continued to surge across much of the U.S., with new single-day infection records reported in nine states. More widespread testing is no doubt playing some role in the increase in the number of known cases. But growing hospitalizations and rising rates of positive tests compared with total tests in many of those states make clear that the virus is raging uncontrolled across much of the Sun Belt.

In Arizona and Texas, more people with the coronavirus are hospitalized now than at any previous point in the pandemic. In Utah, the percentage of positive tests compared with total tests reached the highest levels yet this month. In Nevada, the percentage of positive tests recently began increasing again after more than a month of sustained declines.

Dr. Jeff Duchin, the health officer for Seattle and King County, Wash., said in a statement today that the area had seen a spike in cases in the last week, as it proceeded to loosen restrictions. “As we move into Phase 2 and for the foreseeable future, our risk will be increasing, not decreasing,” Duchin said. “COVID-19 has not gone away and we must take the ongoing risk very seriously.”

In Arizona, Gila River Hotels and Casinos shut its doors again this week after reopening in mid-May with new safety procedures in place. The company said it would close for two weeks “to see whether the recent rise in Arizona COVID cases subsides and to re-examine every aspect of its operation.”

Even as coronavirus cases increase in many states, there was reason for some optimism about the national picture overall. Coronavirus deaths in the U.S. have fallen to roughly 700 a day from a peak of more than 2,000 a day, and some of the country’s hardest-hit regions have showed sustained improvement. New case reports continue to plummet across most of the Northeast and much of the Midwest. The Chicago, Boston, Milwaukee, Detroit and New York areas all continue to improve.

But there were also worrisome signs in those same regions. Case numbers have started trending upward again in Kansas after weeks of falling. The La Crosse, Wisc., area is experiencing a period of explosive case growth. And new trouble spots have cropped up in parts of Missouri, Iowa and Pennsylvania.

As more Americans return to ordinary routines, clusters are emerging in places that had been largely closed. More than 230 cases were tied to a Pentecostal church in Oregon, and far smaller clusters were reported recently at churches in Alabama, Wisconsin and West Virginia. Outbreaks have also been reported recently at a Panda Express restaurant in California, an Advance Auto Parts store in Colorado, and among athletes at Kansas State University and the University of Texas.

South Carolina’s state epidemiologist pleaded with residents to wear masks and practice social distancing as that state identified more than 990 new cases of the virus Thursday. It was the sixth time in 10 days that the state broke its single-day case record.

“We understand that what we’re continuing to ask of everyone is not easy and that many are tired of hearing the same warnings and of taking the same daily precautions,” Dr. Linda Bell, the epidemiologist, said in a statement. “Every day that we don’t all do our part, we are extending the duration of illnesses, missed work, hospitalizations and deaths in our state.”

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