Monster storm Harvey has been downgraded from a hurricane to a tropical storm but is lingering over Texas, US meteorologists said Saturday, warning that it had unleashed "extremely serious flooding."

Harvey was not expected to move much for several days, meaning the state's Gulf Coast was going to see massive amounts of rain, the National Hurricane Center said. Some communities could get up to 40 inches (more than 100 centimeters).

"Rainfall of this magnitude will cause catastrophic and life-threatening flooding," the NHC said.