The trial of four volunteers working in the southern Arizona desert started in Tucson on Tuesday, raising fears about the future of humanitarian work at a time when US President Donald Trump is ratcheting up fear over immigration.

The four No More Deaths volunteers - Natalie Hoffman, Oona Holcomb, Madeline Huse and Zaachila Orozco McCormick - are facing misdemeanours related to leaving behind water containers and other humanitarian supplies on a wildlife refuge, reports The Arizona Daily Star.

The trial is expected to last until Friday.

Established as a coalition of faith and community groups in 2004, No More Deaths leaves out water jugs in the vast desert stretches, searches for migrants who go missing and documents human rights violations by Border Patrol and other US authorities.

Holcomb, Huse, McCormick and Hoffman are accused of entering a national wildlife refuge without a permit and abandoning property there; Hoffman faces an additional charge of operating a motor vehicle in a wilderness area, the Daily Star adds.

Federal prosecutors say the case is straightforward and that the alleged legal violations are clear.

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The incidents allegedly took place on the Cabeza Prieta National Wildlife Refuge, an arid desert stretch where the Pima County Office of the Medical Examiner says at least 43 sets of human remains have been found since the beginning of 2017, as reported by the Daily Star.

Christopher Dupont, a defence lawyer, insisted the volunteers were seeking to help migrants and refugees passing through a "veritable cemetery, a land of unmarked graves".

Speaking to the local Arizona Republic news outlet, No More Deaths spokesperson Justine Orlovsky-Schnitzler described the area as "one of the deadliest corridors for migrant movement".

"Everyone has a right to safe passage and we hope that that's held up [sic] in court," she said.

Fears of crackdown

The trial is the first of a series of legal proceedings against humanitarian aid activists helping migrants and refugees on the southern frontier, including five additional No More Deaths volunteers facing charges stemming from incidents in 2017 and 2018.

Although similar charges have often been dismissed in the past, the Trump administration's hardline anti-immigration stance has many worrying about a crackdown on humanitarian aid work.

Activists and community members held a small rally on Tuesday in Tuscon.

Last month, Trump allowed the government to partially shut down over his unfulfilled demand that Congress approve more than $5bn in funding to erect a wall on the US-Mexico border.

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That shutdown, the longest of its type in US history, has affected more than 800,000 federal employees, who have either been furloughed or are working without pay.

Despite talks, the Democrats have consistently rejected Trump's demand to fund the wall, and Trump has floated the idea of declaring a national emergency to circumvent Congressional approval for the money.

Since coming to office, Trump has sought to restrict the number of refugees and migrants entering the country, deployed thousands of soldiers to the southern border, and falsely described US-bound caravans of people fleeing violence and economic catastrophe as an "invasion".