Greece faces its hottest July weekend in 50 years, with temperatures forecast to soar above 40 Celsius (104 Fahrenheit), while a record-breaking heatwave stretching across the southern United States was expected to expand in the coming days.

Tens of millions of people in the northern hemisphere have been suffering through intense heat this summer as the world appears headed for its hottest July on record.

As temperature records tumble, experts have pointed the finger at climate change driven by the burning of fossil fuels, saying global warming is playing a key role in destructive weather.

Already 11 days into its heatwave, Greece's national weather institute warned reprieve was still days away, setting this up to be the longest hot spell the country has ever seen.

"According to the data, we will probably go through 16-17 days of a heatwave, which has never happened before in our country," Kostas Lagouvardos, the director of research at the National Observatory, told ERT television Saturday.

The previous heatwave record in Greece was set in 1987, when scorching temperatures of over 39C lasted 11 days.

Across the southern United States, about 80 million Americans will swelter in temperatures of 41C and above this weekend, the National Weather Service (NWS) said.

The country's worst heat of up to 46C is forecast for Phoenix, Arizona, which has seen a record-breaking three weeks in a row of highs above 43C.

Tourists, meanwhile, have been flocking to Death Valley National Park, which straddles the border between California and Nevada, to post selfies with a temperature display outside the visitor centre.

Many are hoping to see it break a world record of 56.7C, which was set in July 1913 but was likely the result of a faulty measurement, according to several meteorologists.

Regardless, July 2023 is on track to be the hottest absolute month -- not only since records began, but also in "hundreds, if not thousands, of years," leading NASA climatologist Gavin Schmidt said.

The effects cannot be attributed solely to the El Nino weather pattern, which "has really only just emerged" and isn't expected to strengthen until later on in the year.

El Nino is associated with the warming of ocean surface temperatures in the central and eastern Pacific Ocean.

Schmidt said the trend of extreme heat was expected to persist, "and the reason why we think that's going to continue, is because we continue to put greenhouse gasses into the atmosphere".

Greece, which is battling dozens of forest fires, warned people not to venture out unnecessarily due to the baking heat.

The exceptional temperatures also mean key tourist sites such as the Acropolis will be closed during the hottest part of the day.

"This weekend risks being the hottest registered in July in the past 50 years," said Panagiotis Giannopoulos, meteorologist with state broadcaster ERT. (AFP)