The US southwest is baking under an unrelenting heatwave for a fifth day, putting power systems to the test as more records were expected to be broken before a moderating trend sets in next week.

The US National Weather Service issued excessive heat warnings for five states - California, Nevada, Utah, Arizona and part of Colorado.

The heat has been extreme - even for a region where temperatures often exceed 38C (100F).

"Every year it's hot in the southwest," said meterorologist Bob Oravec of the National Weather Service Weather Prediction Centre in College Park, Maryland.

"It just seems to be more newsworthy when you have temperatures of 115 or so day after day. It's pretty hot."

The heatwave stressed power systems in Texas and California this week, but they withstood the record demand.

Power grid operators in both states, however, continued to urge consumers to conserve energy to reduce strain on the electric system and avoid outages as homes and businesses kept air conditioners cranked up.

People cool off at the Six Flags Hurricane Harbour water park in Concord, California

In Texas, where temperatures have moderated, demand hit a record last Monday, according to the Electric Reliability Council of Texas, which operates the grid in most of the state.

California's power demand peaked yesterday at 41,364 megawatts and was expected to come in close to that level today, according to the California ISO, which operates the gridin most of that state.

One megawatt can power about 200 homes on a hot summer day.

Among the cities where record highs were expected to be set today were Phoenix at 47C (117F) and Sacramento, California, at 42C (108F), the weather service said.

Many others were expected to come close to tying or breaking their daily records, including Las Vegas where a high temperature of 46C (114F) was forecast.

The heatwave extended to the midwest, prompting the weather services to issue advisories for Kansas, Missouri and Illinois, before a strong cold front brings relief by the end of the weekend.

Temperatures in St Louis and Kansas City, Missouri were forecast to top 38C on Friday.

Relief, relatively speaking, is also forecast to come to the southwest at the beginning of next week, Mr Oravec said.

"Basically, you're going to have temperatures in the low hundreds, but definitely a cooler trend, a little bit, as we go into early next week," he said.