Hot weather alerts are in place for more than 30 million people across the western US after the region's second heat wave in weeks brought another round of record-equalling high temperatures.

Sweltering conditions have hit much of the Pacific seaboard and as far inland as the western edge of the Rocky Mountains over the weekend, with forecasters warning of more to come on July 11.

Las Vegas matched its all-time record of 47.2 degrees Celsius, according to the National Weather Service (NWS) – a temperature recorded in the desert entertainment city once in 1942 and three other times since 2005.

Forecasters issued an excessive heat warning for the city along with several other urban centres including the southern city of Phoenix and San Jose, the centre of the Silicon Valley tech industry south of San Francisco.

Last month was the hottest June on record in North America, according to data released by the EU's climate monitoring service.

Human activity has driven global temperatures up, stoking increasingly fierce storms, extreme heatwaves, droughts and wildfires.

The World Meteorological Organisation and Britain's Met Office said in May there was a 40 per cent chance of the annual average global temperature temporarily surpassing 1.5C above pre-industrial temperatures within the next five years.

The past six years, including 2020, have been the six warmest on record.