With more than 300 students at the University of Missouri testing positive for the coronavirus and an alarming infection rate of 44 per cent for the surrounding county, the local health director on Friday ordered bars to stop serving alcohol at 9pm (local time) and close by 10pm.

Iowa's governor has ordered all bars shut down around The University of Iowa and Iowa State, while the mayor of Tuscaloosa, Alabama, did the same in the hometown of the state's flagship university.

"What we're seeing in our violations is they're coming late at night," said Stephanie Browning, head of the health department for Columbia, Missouri.

"Big groups gathering. They're not wearing their masks, they're not social-distancing."

In Utah, the city council in Provo overrode a mayoral veto to pass a mask mandate, which includes a $US500 ($A681) fine for organizing large mask-less gatherings, days before students return to Brigham Young University from around the country.

But the council was nevertheless worried that college students sharing dorms, bathrooms and showers could become "super-spreaders" and bring the virus off campus, council chair George Handley said.

"We're already seeing what's happened across the country at other universities," he said.

The outbreaks since students began returning to campus in the past few weeks have heightened tensions between colleges and their towns and led to recriminations between local politicians and university officials.

Meanwhile, California announced a four-tiered, color-coded plan Friday for gradually reopening businesses after abandoning a reopening attempt earlier this summer. It requires counties to meet certain benchmarks showing progress in controlling the virus.

In Arizona, another deadly hot spot this summer, a drop in transmission numbers allowed the Phoenix and Tucson areas to reopen gyms and some bars.

The U.S. has recorded over 180,000 deaths from the coronavirus and 5.9 million confirmed infections. Worldwide, the death toll is put at more than 830,000, with at least 24.5 million cases.

Drugmaker Gilead Sciences said on Friday that US regulators were allowing use of experimental antiviral drug remdesivir for all patients hospitalized with COVID-19. Until now, that was limited to patients with severe COVID-19.

Surging infection numbers around the US have been blamed in part on young people ignoring mask and social distancing requirements.

In Iowa's Story County, home to Iowa State, 74 per cent of new cases over the past seven days were among people ages 19 to 24, Governor Kim Reynolds said Thursday. In the same time period, 69 per cent of new cases in Johnson County, the home of the University of Iowa, were in that age group.

"It is increasing the virus activity in the community, and it's spilling over to other segments of the population," Reynolds said.

Fueled in part by clusters where college students are returning to classes, Kansas has had its highest seven-day increase in coronavirus cases.

Governor Laura Kelly said outbreaks on college campuses and fraternities and sororities are at least a factor in the surge.

"We cannot continue to go down this path," she told reporters Friday.

Australian Associated Press