At least 25 people have died in a weeklong heat wave in the US, NBC News reported Sunday, as 40 million people remained under heat alerts across the East Coast, southeast and southwest.

According to state and local officials, New Jersey has reported 22 suspected heat-related deaths, while Illinois reported one fatality and Mississippi reported two.

The National Weather Service (NWS) warned that thunderstorms could bring damaging winds, hail and localized flash flooding across parts of the East Coast through Monday.

Flood alerts remain in effect for 34 million people from Delaware to Connecticut as well as New York City, where forecasters expect up to 3 inches (7.6 centimeters) of rainfall.

Severe storms have also caused power outages affecting hundreds of thousands of customers across several eastern states.

The NWS said heat alerts would continue into Sunday evening across the East Coast, with heat index values — which reflect how hot it feels when humidity is combined with air temperature — expected to reach between 100F (37.7C) and 105F (40.5C) in Philadelphia, Washington DC, Baltimore, Raleigh, Charleston, South Carolina and Jacksonville, Florida.

Forecasters said temperatures across much of the East Coast are expected to gradually ease this week, with daytime highs generally ranging from the 70s to the low 90s.

However, dangerously hot conditions are forecast to persist through midweek in parts of the country. Extreme heat watches have been issued for parts of California and Arizona, including Phoenix and Tucson, from Tuesday through Thursday, with daytime highs expected to reach as high as 114F (45.5C).

In New York City, more than 378 people have visited emergency rooms for heat-related illnesses, according to the city's health department. (Anadolu Agency)