Students enjoy daytime outdoors on the campus of Columbia University, New York, the United States, March 10, 2020. (Xinhua/Wang Ying)

Students are returning to university campuses across the United States amid an increase in new COVID-19 cases, adding challenges to the country's control of the widely spread virus.

Whether and how to reopen schools in the United States this fall has become a hotly-debated issue in recent weeks. While some politicians have been pushing for in-school learning as soon as possible, many families are hesitant to send their children back due to safety concerns.

Several universities have already detected new COVID-19 cases. Earlier this week, the University of North Carolina at Chapel Hill, one of the largest schools in the country to bring students back to campus and attempt in-person teaching, announced it will suspend in-person instruction for undergraduates in a dramatic turnaround just a week after classes began.

The University of Notre Dame is also shifting to online classes after reporting a rise in infections.

"Our contact tracing analysis indicates that most infections are coming from off-campus gatherings. Students infected at those gatherings passed it on to others who in turn passed the virus on to a further group, resulting in the positive cases we have seen," said the university's president John Jenkins.

Photo taken on July 14, 2020 shows a view of the campus of Harvard University in Cambridge of Massachusetts, the United States. (Photo by Fan Lin/Xinhua)

Los Angeles Unified, the second-largest school district in the nation, plans to periodically test hundreds of thousands of students and 75,000 employees for the virus to gain clarity on when in-person instruction can resume safely.

"Extraordinary circumstances call for extraordinary actions, and while this testing and contact tracing effort is unprecedented, it is necessary and appropriate," said Austin Beutner, superintendent of Los Angeles Unified.

As new COVID-19 cases are reported at K-12 schools that have reopened, many others are facing the difficult decision of whether or not to return to in-person classes.

Medical experts warn it could take years before students and teachers can return to in-person education safely without masks, social distancing and other measures intended to curb the spread of the coronavirus.

A latest report released by the American Academy of Pediatrics and the Children's Hospital Association showed 75,755 new child cases reported from July 30 to Aug. 13, a 24 percent increase in child cases over two weeks.

"While children represented only 9.1 percent of all cases in states reporting cases by age, over 406,000 children have tested positive for COVID-19 since the onset of the pandemic," said the report.

Zhang Zuofeng, a professor of epidemiology and associate dean for research with the school of public health at University of California, Los Angeles, told Xinhua that the increase in infections among children may be related to such factors as more gatherings of younger ones and going back to school during the pandemic.

A man walks past buildings of New York University in New York, the United States, July 14, 2020. (Xinhua/Wang Ying)

The US Centers for Disease Control and Prevention (CDC) has been stressing the importance of returning to school, saying the best available evidence indicates that COVID-19 poses relatively low risks to school-aged children. Death rates among school-aged children are much lower than among adults.

"At the same time, the harms attributed to closed schools on the social, emotional, and behavioral health, economic well-being, and academic achievement of children, in both the short- and long-term, are well-known and significant," said the CDC on its website.