A massive heat dome engulfing the southern US is set to roast seven US states for days as temperatures in some areas soar past 110 degrees.

More than 50 million people living in Arizona, Arkansas, California, Louisiana, New Mexico, Oklahoma and Texas have been placed under extreme heat warnings or heat advisories by the National Weather Service (NWS) on Monday.

Texas is forecasted to be the hardest hit, with Dallas, Fort Worth, Houston, San Antonio, Austin, Lubbock, Odessa and El Paso all impacted by the high-pressure system blanketing the Southwest. 

Phoenix and Tucson in Arizona, San Diego and Los Angeles in California, Shreveport in Louisiana and Oklahoma City, Oklahoma, have all been warned of the potentially deadly combination of extreme heat and high humidity lasting until at least 8pm local time tonight.

In California, the heat warnings will remain in effect until Wednesday, and in Arizona, the life-threatening weather is expected to last until Saturday night - extending the risk of residents experiencing heat exhaustion or heat stroke.

A spokesman for the NWS said: 'An extreme heat warning means that a period of very hot temperatures, even by local standards, will occur.'

Temperatures in the Phoenix area are expected to reach between 106 and 118 degrees Fahrenheit in the afternoons this week.

In Texas, forecasters are bracing for near-record temperatures that could reach 112F in multiple areas on Monday, affecting roughly 25 million people in that state alone.

NWS described the conditions in Texas on Monday as 'major' or 'extreme' on their heat risk guidance chart - the two most serious levels on the five-stage alert system.

Extreme describes weather where there will be 'no overnight relief' from the heat, and conditions that will affect anyone who does not have air conditioning or fails to hydrate properly while outdoors. 

Dallas, Fort Worth, Austin and Houston have all been listed in the extreme heat zone on Monday.

By Wednesday, the worst of the heat and humidity has been projected to shift west, towards Arizona and Southern California, impacting Phoenix, Tucson and Los Angeles.  

NWS officials urged: 'Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors.'

'Do not leave young children and pets in unattended vehicles. Car interiors will reach lethal temperatures in a matter of minutes,' they added in their alert.

The forecasted high on Tuesday (114F) and Wednesday (116F) in Phoenix is expected to break or challenge the city's single-day records, which were set for both days back in 2011.

Multiple heat domes have formed over the US this year, starting unseasonably early in mid-March. Heat domes prevent both hot air and high humidity levels from rising into the atmosphere, acting like a lid over large portions of the US for days at a time.

For those who have to go outside, officials urged Americans to wear lightweight and loose-fitting clothing and limit strenuous activities to early morning or evening hours.

The elderly, pregnant women, newborns, young children and those with chronic health problems such as heart disease, obesity or poor blood circulation are said to be at the highest risk from heat-related illnesses.

Signs of heat exhaustion, when the body overheats from prolonged exposure to hot weather, include heavy sweating, fatigue, developing cool, pale, clammy skin and a fast, weak pulse, muscle cramps, dizziness, nausea and headaches.

Heat stroke, which is far more deadly and can develop within 15 minutes in extremely hot conditions, often starts with a throbbing headache, confusion, slurred speech, nausea, dizziness and a fever above 103F.

Sufferers may also develop red, dry, or damp skin, a rapid pulse, and lose consciousness. It turns fatal when the body's organs begin to fail.

The conditions in the Southwest are expected to exacerbate an already extreme drought throughout the region, which may soon lead to water shortages in several states.

The US Drought Monitor has noted that conditions in northern Texas, western Oklahoma and parts of New Mexico have reached the monitor's most severe level of drought, called D4 or 'exceptional drought.'

Large portions of nearby Colorado, Idaho and Oregon have also moved into this most severe drought status - increasing the risk of deadly wildfires throughout the western US this summer.