Temperatures in California's Death Valley reached 54.4C on Sunday, possibly the highest ever reliably recorded on the planet.
The preliminary reading was noted by the US National Weather Service's automated weather station at Furnace Creek at 3.41pm.
Death Valley - a strip of land between two mountain ranges - is the lowest, driest and hottest location in the US and one of the hottest places on Earth.
The valley's Greenland Ranch hit 56.7C in 1913, but some experts question the accuracy of old temperature reports. Sunday's reading will now be investigated by the National Centre for Environmental Information and the World Meteorological Organisation.
If verified, it would be the hottest August day ever recorded in the US, and the third-highest temperature ever recorded, after readings in 1913 and 1931.
Some weather watchers believe it could prove to be the hottest temperature ever reliably recorded on the planet.
Bob Henson, a meteorologist, said: "It's quite possible the Death Valley high set a new global heat record. The extreme nature of the surrounding weather pattern makes such a reading plausible, so it deserves a solid review. There are nagging questions about the validity of even hotter reports from Death Valley in 1913 and Tunisia in 1931.
"What we can say with high confidence is that, if confirmed, this is the highest temperature observed on Earth in almost a century."
Much of the western US has been gripped by suffocating heat since late last week. The record temperatures have caused a number of fires across California.
It was a dry heat: humidity fell to 7pc. But it felt "insanely hot" all the same, according to meteorologist Daniel Berc, who is based in the National Weather Service Las Vegas bureau and forecast that the heatwave would continue all week. "It's literally like being in an oven," he said. (© Daily Telegraph, London)
Telegraph Media Group Limited