An oppressive heatwave has gripped much of the United States, bringing record-high temperatures that are expected to spread from the nation's midwest to the east coast.

A combination of hot temperatures and high humidity has pushed heat indices in some areas upwards of 51 degrees Celsius.

About 40 states across the country reached highs of at least 32 degrees Celsius in some areas on Tuesday.

The National Weather Service has issued heat watches, warnings and advisories for at least 17 states where the high indices have left people especially vulnerable. Authorities in many areas have set up cooling centers in public buildings and malls for residents to escape the heat.

Forecasters say high pressure in the upper level of the atmosphere will cause the hot temperatures to linger for several days.