Heat dome's triple-digit temperatures fry US south-west

Firefighters are on standby to submerge heat-stroke victims in ice and some popular hiking trails have closed in Arizona as tens of millions of people across the south-western US swelter in record-breaking, triple-digit temperatures.

Two weeks before summer even officially starts, excessive heat warnings were in effect across parts of California, Nevada, Arizona and Texas. Forecasters see no relief for several more days.

Temperatures were predicted on Wednesday to reach 109F (42.7C) in Phoenix, 107F in Las Vegas, 110F in Palm Springs and 119F Death Valley, California.

By end of the day, the National Weather Service (NWS) says Americans in the region may experience "easily their hottest" weather since last September, according to the Associated Press.

The mercury is soaring as a result of a heat dome, an area of high pressure where hot air is pushed down and trapped, causing temperatures to soar over large areas.

Temperatures will be 20-30 degrees Fahrenheit above average for this time of year, according to the NWS.

In Arizona, the hottest big city in the US, firefighters have placed at least one heat-stroke victim inside human-sized immersion bags filled with ice cubes to lower the patient's body temperature on the way to hospital.

All Phoenix fire department vehicles are being equipped with the bags.

There were 645 heat-related deaths last year in Maricopa County, where Phoenix is located.

The city is also opening two overnight cooling stations this week for the first time.

The NWS predicted temperatures could reach 111F at the Grand Canyon and advised that hikers use extra caution when outdoors for an extended period of time at lower elevations.

The excessive heat has also led to Arizona officials closing popular trails at Camelback Mountain and Piestewa Peak.

Forecasters predict temperatures in Las Vegas could reach 112F on Thursday.

Across the state, temperatures were expected to range from 102F to 115F.

In Texas, San Angelo reached 111F on Tuesday, tying with the fifth hottest temperature ever recorded in that city, according to the local NWS office.

The heat warnings were expected to run until the end of Friday.

The extreme temperatures are forecast to spread north by the weekend to the Pacific north-west.