At least four people have died crossing the US-Mexico border near El Paso, Texas, amid the searing heatwave gripping the south-west.

Temperatures in El Paso peaked at 106F (41C) on Thursday, and some 34 million people – from the southern tip of Texas across Arizona and up into California and Nevada – were under heat alerts.

The US border patrol in El Paso said it had identified four people who died last weekend from “heatstroke and dehydration”. At least two others who are presumed to be migrants died this week, US Customs and Border Protection told the Guardian – though the individuals’ identities and causes of death have yet to be confirmed by the local coroner’s office.

The extreme heat across the south-west has come earlier than usual this year and experts have warned this could be a record-setting season. Last year, amid unprecedented heatwaves across the region, the CBP El Paso sector recorded 686 deaths and disappearances – which was the highest-ever toll the agency had recorded. “As temperatures soar and summer approaches, the treacherous conditions of the desert are proving increasingly dangerous,” the agency wrote in a statement.

Advocates for migrants have warned the new restrictions on asylum access ordered by Joe Biden this week will further push migrants to take riskier journeys across the border in brutal conditions.

“At a time that we’re seeing record-high heat in the region, enacting an executive order that basically shuts down the border with little notice is another way that our policies put people at risk,” said Javier Hidalgo, legal director at Raices, an immigrant support and advocacy group in Texas.

Increasingly, many of those attempting to cross into the US are fleeing extreme weather and climate disasters in their home countries. US policies at the border, Hidalgo added, “are not centering human life”.

The actual number of migrant deaths in the region may be up to four times higher than official counts, according to a report released in March by the aid group No More Deaths, which provides water and first-aid care to migrants. The group collected data on deaths through public records requests to various medical investigator’s offices, news reports and public data provided by CBP and other agencies. The report ascribes the undercount to a lack of follow-ups with hospitals, local law enforcement and medical examiners offices after border patrol agents find people who are injured or dead.

Although the extreme heat in the Chihuahuan and Sonoran desert has always been treacherous, researchers at the University of Arizona have challenged the idea that the heat alone has led to growing numbers of migrant deaths. Migrants who cross the border without authorization have found safer, shaded routes through mountain regions, researchers found in a 2022 paper.

But due to US policy decisions – including ones that have limited access to asylum and other legal paths into the US – migrants have been “compelled to undertake an increasingly lengthy, difficult and physiologically taxing journey” that has often proved deadly, the researchers conclude.

On Tuesday, the president signed an immigration order that temporarily suspends access to the asylum system for those crossing the border between official ports of entry when the daily number of crossings exceeds 2,500.

“Just because there’s a new rule doesn’t mean that people are not going to try to attempt to cross,” said Alvaro Huerta, director of litigation and advocacy at the Immigrant Defenders Law Center. But they may be more likely to take more remote trails. “So especially over the heat of the summer, when people are crossing over dangerous terrain, they’re much more likely to die of dehydration or an injury,” he said.