Much of the western United States is braced for record heat waves this week, with approximately 50 million Americans placed on alert Tuesday for "excessive" temperatures, which could approach 120 degrees Fahrenheit (50 degrees Celsius) in some areas.

"A prolonged and record-breaking heatwave is underway across the western US," said the US National Weather Service, with "above normal to extreme high temperatures" expected at least through the weekend.

The heatwave, which stretches like a blob over much of the western third of the US map, extends east to west from Wyoming to California, and north to south from Idaho to Arizona.

Average temperatures in the vast region are around 20 degrees Fahrenheit above the seasonal norm, with the arid desert states of Arizona and Nevada most likely to shatter records.

The Arizona city of Phoenix experienced temperatures of 115 degrees Fahrenheit on Monday but forecasters predict it could pass 118 degrees by the end of the week, with lows in the upper-80s at night.

Las Vegas in neighbouring Nevada is suffering its most extreme prolonged heatwave since early summer 2013, according to local media, and the city by Wednesday could near its decades-old temperature record of 117 degrees Fahrenheit, set in 1940.

Authorities have opened "cooling stations" for vulnerable residents without access to air conditioning and called on social media "influencers" to help warn the public about the impact of the sweltering heat on pets.

The highest temperature ever officially recorded was in nearby Death Valley, at 131.4 degrees Fahrenheit back in 1913.

As well as warning that current temperatures are "rare, dangerous and deadly," officials fear that the heat -- arriving so early in the year -- could lead to deadly forest fires, aggravated by chronic drought in the American West.

Some 88 percent of the region is in a state of drought, including all of California, Oregon, Utah and Nevada, according to the latest government surveys.