More U.S. colleges were grappling with high numbers of students testing positive for the coronavirus just days into the start of the fall semester after some universities rolled back their campus reopening plans in recent weeks.

The University of Alabama on Monday reported more than 550 people across its campuses had tested positive for COVID-19 since it resumed in-person classes on August 19. Most of those infected were students, faculty and staff at the university's main campus in Tuscaloosa.

Citing a "dramatic increase" in coronavirus cases on campus, the mayor of Tuscaloosa issued an executive order on Monday ordering bars to shut down for 14 days and placing restrictions on other establishments.

"Many students who tested positive for COVID-19 have chosen to go home to isolate," Kellee Reinhart, the university's vice chancellor for communications, told Reuters in an email.

The university has conducted more than 46,000 tests, according to a dashboard it released this week, and the positivity rate stood at about 1 percent. The number of positive cases does not include the 400 students who tested positive upon returning to University of Alabama campuses before classes began last week.

Alabama is not alone in scrambling to deal with COVID-19 college outbreaks.

The University of Southern California (USC), which resumed education almost entirely online on August 17, on Monday said that more than 100 students at the University Park Campus in Los Angeles were in a 14-day quarantine after exposure to the virus.

"USC Student Health has received an alarming increase in the number of COVID-19 cases in students in the University Park Campus community," the university said in a statement, adding that all cases were related to students in "off-campus living environments."

Ohio State University, where classes resume on Tuesday, this week issued more than 200 interim suspensions for students following a string of large parties where health and safety rules were largely ignored, according to media reports.

Last week, the University of North Carolina at Chapel Hill canceled in-class instruction after positive cases of COVID-19 shot up dramatically.

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Young people fueling recent spike in Americas

According to the Pan American Health Organization (PAHO), young people are driving the spread of the coronavirus in the Americas, considering that both deaths and caseloads have doubled in the region over the past six weeks.

"The vast majority" of reported COVID-19 cases in the Americas have been among those between the ages of 19 and 59, Dr. Carissa Etienne, head of PAHO, told a webcast briefing Tuesday, saying this indicates that younger people are primarily driving the spread of the disease in the region.

Overall, governments should base their re-opening decisions on the best available data and expand testing and contract tracing programs to better identify and control spikes in cases, she said.

Six of the world's 10 most affected countries are in the Americas, said Etienne, pointing to the United States, Brazil, Mexico, Colombia, Peru and Argentina.

(With input from Reuters)