A searing heatwave brought summer misery to the American heartland and caused more than a dozen deaths as it blanketed up to 17 US states in soaring temperatures and oppressive humidity.

The weather is responsible for at least 13 deaths in the central United States, which has seen a string of days with temperatures topping 100 degrees Fahrenheit (37.7 Celsius), according to officials.

The heat has been coupled with near record humidity -- a miserable combination that forecasters said would persist into next week.

A National Weather Service spokesman told AFP that more than 1,000 heat records had been broken across the United States in the past month.

Across the Great Lakes region, the most extreme temperatures are expected to arrive Wednesday and Thursday.

The central US states of Illinois and Indiana were expecting temperatures and humidity indices above 115 degrees Fahrenheit (46 Celsius), with even worse weather predicted in some areas by Thursday.

Monday's heat index reached 124 degrees Fahrenheit (51 Celsius) in the northwestern Illinois town of Freeport, according to the NWS, and even topped a tropical 130 degrees Fahrenheit (54 Celsius) in parts of Iowa.

After dipping slightly on Tuesday, the heat is expected to return with a vengeance to Chicago, where the heat index could reach and stay above the 105 degree-mark (40 Celsius) -- a relatively rare occurrence, according to the National Weather Service.

Chicago officials are urging area residents, especially the elderly, the very young and those with health conditions to take the threat of heat seriously.

Adding to the region's weather woes, the NWS reports that severe thunderstorms accompanied by large hail and very high winds are possible daily through the weekend.

In Oklahoma, intense high temperatures are reported to have caused roads in the southern plains state to buckle, while dozens of people required medical treatment, according to the Daily Oklahoman newspaper, which reported that on Monday Oklahoma City saw its 28th day of triple-digit temperatures this year.

Meanwhile, residents of normally temperate Minnesota, unaccustomed to extreme heat, broiled in sweltering heat which, combined with the humidity, felt like 122 degrees Fahrenheit (50 Celsius).

Forecasters said high pressure in the upper atmosphere ensured that the searing weather would remain in place in the heartland for days on end, and would not spare the eastern United States.

The US east coast already Tuesday was beginning to feel the effects of the oppressive weather, which was expected to reach New York, Washington DC and the rest of the eastern seaboard full force by midweek.

Each year heat kills 162 people on average in the United States, more than hurricanes (117 on average), floods (65), tornadoes (62) or lightning (48).

Just last month, NOAA released data showing that 30-year temperatures across the United States had risen appreciably -- by some 1.5 degrees Fahrenheit.

"The climate of the 2000s is about 1.5 degree Fahrenheit warmer than the 1970s," said Thomas Karl, director of NOAA's National Climatic Data Center.

The 30-year baseline is used by scientists to understand climatic trends, including climate change.

Further west, temperatures were nearer to normal in states like California and Arizona, although there were red flag fire alert warnings in parts of Nevada and Utah, according to the National Weather Service.

In southern California, which hit a record 113 degrees Fahrenheit (45 Celsius) in downtown Los Angeles last September, temperatures were expected to peak in the high 80s (high 20s Celsius) on Tuesday -- perfect beach weather for the Golden State.

But authorities nevertheless felt the need to open "cooling centers" for residents not lucky enough to have air conditioning -- even as it urged those who do to use it sparingly, so as not to tax the state's over-extended electrical grid.

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