WASHINGTON - Leading epidemiologists warned on Monday that Midwest regions in the United States, which saw sharp increase of COVID-19 infections recently, might be another "hotspot" for the pandemic.

The total number of COVID-19 cases in the United States surpassed 6 million on Monday, with national death toll reaching over 183,400, according to a tally of Johns Hopkins University.

"Nationwide, metrics on new cases, deaths, hospitalizations and the positivity rates of tests are all declining, but there are emerging hotspots in the Midwest," Zhang Zuofeng, professor of epidemiology and associate dean for research with the school of public health at University of California, Los Angeles, told Xinhua.

"Moreover, the back-to-school season has brought new challenges to the control of the pandemic," he added.

Indicators that track influenza-like illness and COVID-19-like illness and the percentage of laboratory tests positive for SARS-CoV-2 have continued to decrease nationally since mid-July, according to a latest report of the US Centers for Disease Control and Prevention.

Weekly hospitalization rates and mortality attributed to COVID-19 declined over last week, said the report.

In the Midwest, however, Iowa, North Dakota, South Dakota and Minnesota have recently reported record one-day increases in new cases, while Montana and Idaho are seeing record numbers of currently hospitalized COVID-19 patients.

Many of the new cases in Iowa are in the counties that are home to universities, which are holding some in-person classes.

Colleges and universities across the United States have seen increasing outbreaks as students are returning to campus, forcing some to switch in-person instruction to online courses.

Deborah Birx, the White House coronavirus coordinator, recently said that universities should implement entry tests and be prepared to test up to 10,000 people a day, the Center for Public Integrity reported.

"We have already seen that if schools do not do testing and quarantining, the number of cases will spike at universities," Stanley Perlman, professor of microbiology and immunology at the University of Iowa, told Xinhua.

"The same could happen in younger children, although they are generally asymptomatic and schools are often online in the country now," he added.

Some midwestern states never locked down and others opened up perhaps too early, Perlman said, adding he fears that there will be hotspots in other places when fall and winter come.