Hot and dangerous: US bakes under relentless heat dome

Authorities sound alarm, warn of possible fatal dehydration amid soaring mercury

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Brutally high temperatures threatened tens of millions of Americans on Saturday, as numerous cities braced themselves for new temperature records under a relentless heat dome that has baked parts of the country all week.

The National Weather Service (NWS) warned of an "extremely hot and dangerous weekend", with daytime highs routinely ranging between 5.5 deg C and 11 deg C above normal in the US West.

Residents of central and southern California, for example, could expect to see thermometers peaking at 38 deg C to 43 deg C, it said.

The heat is forecast to remain anchored over the West for the weekend, "growing hotter in the South by early next week".

The authorities have been sounding the alarm for days, advising people to avoid outdoor activities in the daytime and to be on the watch for signs of dehydration, which can quickly become fatal in such temperatures.

In the hardest-hit areas, residents face a daily endurance marathon against the sun.