A relentless heat wave has killed at least 40 people in the south-western region of the United States, including nearly two dozen migrants trying to cross the border from Mexico.

The United States Customs and Border Patrol says in the month of July at least 21 migrants died from heat-related causes while trying to pass through the sweltering desert border.

Meanwhile, officials in the southwestern U.S. state of Arizona recently confirmed that temperatures soaring past 43 degrees Celsius have killed around 20 people in Phoenix since July 1. Authorities say homeless people and transients were among the majority of the death count.

Homeless shelters there are staying open to allow vulnerable populations escape the severe heat, while officials also have been distributing items like sunscreen and hats.

Some information for this report provided by AP and AFP.