Washington, Sep 29 (IANS): At least 21 US states reported an increase in the number of confirmed Covid-19 cases, as health experts have warned against a surge in the virus spread in the upcoming fall and winter seasons.

The 21 states are Alabama, Alaska, Colorado, Idaho, Maine, Michigan, Minnesota, Montana, Nevada, New Jersey, New Mexico, North Carolina, North Dakota, Oregon, South Carolina, South Dakota, Texas, Utah, Washington state, Wisconsin and Wyoming.

A CNN analysis revealed on Monday that the number of new cases has increased by at least 10 per cent or more compared to the week before in the affected states as of Sunday.

Chris Murray, director of the University of Washington's Institute for Health Metrics and Evaluation (IHME), said that the US, the worst-hit country presently, was likely to witness an explosion of Covid-19 cases in the fall and winter when people spend more time indoors, leading to a greater risk of transmission.

A recent IHME model has showed a "huge surge" expected in October "and accelerate in November and December".

The model indicated that single-day Covid-19 deaths in the US could jump to 3,000 by December, than the present average of about 765 fatalities.

Previously, Robert Redfield, Director of the US Centers for Disease Control and Prevention, had previously warned that the two coming seasons could be "one of the most difficult times that we experienced in American public health", CNN reported.

Last week, the US' top infectious disease expert Anthony Fauci said with daily cases averaging 40,000 nationwide, the new season could be a challenge.

"You're going to have to do a lot of things indoors out of necessity of the temperature, and I'm afraid, with that being the case, if we don't carefully follow the guidelines ... the masking, the distance, the crowds, that we may see another surge."

As of Tuesday, US still maintained the world's highest number of cases and deaths at 7,147,751 and 205,062, respectively, according to the Johns Hopkins University.