US President Donald Trump has urged universities to reopen their campuses, despite reports of COVID-19 cluster outbreaks among students across the country.

The virus is very dangerous for older people, "but for university students, the likelihood of severe illness is less than or equal to the risk of a seasonal flu", Xinhua news agency quoted Trump as saying to CNBC news on Thursday.

However, some universities are rethinking their plans to reopen for the fall semester after several outbreaks emerged soon after students returned to campus.

The universities, including the University of North Carolina at Chapel Hill, the University of Notre Dame, and Michigan State University, have decided in succession to halt the majority of in-person classes for the fall.

"Instead of saving lives, the decision to close universities could cost lives. It is significantly safer for students to live with other young people than to go home and spread the virus to older Americans," Trump said.

The number of COVID-19 cases among young people in the US has increased sharply recently.

Leading experts told Xinhua on Wednesday that factors including high Covid-19 infections among adults, increasing gatherings among teenagers and returning to school during the pandemic, may be contributing toward the high infection rates.

As of Friday, the US accounted for the world's highest number of cases and deaths at 5,573,501 and 173,114, respectively.