As the early season heat wave in the United States expands in many parts of the country, over 100 million people have been alerted due to the extreme heat.

The U.S. National Weather Service (NWS) said the heat wave would continue through the week as it expands "from the Plains into the Midwest and Southeast."

"Much above normal record-tying and record-breaking high and low temperatures are forecast. The warm lows will provide little relief from the heat overnight," the NWS said on Sunday.

It said in a separate weather alert that "dangerous heat" is expected to extend from the Mississippi Valley to the Southeast through the middle of the week, and "critical fire weather conditions" would be present in the southwest and central Rockies through the Monday night.

High temperatures are expected to hit the 90s and 100s degrees Fahrenheit (30s and 40s degrees Celsius) "with heat indices likely to be in the upper 90s and 100s."

Excessive heat warnings have been issued for at least 11 states spanning the Midwest, South and eastern sea board while heat advisories have been issued for about a dozen other states.

Multiple reports said the number of people that are being affected surpasses 100 million.