At least 14 people have died from the extreme heat wave that has been tormenting the southern United States for two weeks, officials said Friday, with air in other parts of the country polluted by forest fires in Canada.

The highest death toll, 12 people, was registered in Webb County, Texas, near the Mexican border.

"As of Wednesday, there have been 12 total deaths," local officials said in a statement to AFP. "Eleven are Webb County residents, the twelfth death was from a neighbouring county that was brought to a local hospital and unfortunately passed away."

A 14-year-old died last week when he was hiking in Big Bend National Park in Texas, where temperatures reached 119 Fahrenheit (48 Celsius). Tragically, the victim's stepfather died in a car accident as he was rushing to the boy's rescue.

And a 62-year-old woman died in the neighbouring state of Louisiana last week, after a storm left thousands of families without power and thus without air conditioning, according to local officials.

In recent days, temperatures in some southern US cities have felt like 113 degrees Fahrenheit, with the pavement cracking in Houston, Texas and authorities setting up cooling centers in the city of with 2.3 million.

Meanwhile, Canada continued to battle the worst forest fire season in its history, a phenomenon that scientists say is exacerbated by human-induced climate change.

As smoke drifted south, large parts of the United States that are home to more than 120 million people, from the Midwest to the East Coast, remained under air quality alerts.

In New York and Philadelphia, the air was considered unhealthy, according to the government platform AirNow.

Air quality alerts were also issued in the Canadian province of Ontario, as well as for much of the North American Great Lakes and parts of Minnesota, North Carolina and Georgia.

Smoke from the wildfires has also drifted across the Atlantic Ocean and over European countries, including Portugal and Spain.