WASHINGTON, United States (AFP) — A dangerous heatwave is expected to sweep across much of the central and eastern United States (US) beginning Tuesday and lasting through the July 4 weekend, when the country celebrates its 250th birthday and hosts knockout stages of the World Cup.

Temperatures are forecast to soar above 100F (38C), with high humidity pushing heat indexes even higher, the National Weather Service warned Monday.

Overnight cooling will be limited, with many areas seeing lows in the mid- to upper-70s, the agency added.

That combination of prolonged daytime heat and little nighttime relief is expected to increase the risk of heat-related illness, particularly for the most vulnerable.

“Drink water often. Limit time outside in the afternoon. Check on neighbours,” said the public alert system in the nation’s capital, Washington.

The heatwave arrives as the United States prepares to mark the 250th anniversary of its independence from Britain, with the capital set to host what organisers say will be the largest fireworks display in history.

The World Cup has meanwhile entered its knockout stages, with several matches set to take place in the sweltering conditions.

More frequent, longer-lasting and more intense heatwaves are one of the clearest signs of climate change globally, with Europe also recently hard hit.