United Nations human rights experts on Wednesday urged U.S. authorities to drop charges against an Arizona man on trial for providing aid and shelter to migrants crossing over from Mexico.

A prosecutor said Tuesday at the trial that Scott Warren, an activist in the southwestern U.S. state charged with harbouring two undocumented migrants, was part of a conspiracy to transport and “shield” the men from law enforcement.

Warren faces three felony counts for allegedly transporting, harbouring and hiding the men — a case likely to set a precedent over what aid U.S. citizens can give to those who cross the border between ports of entry. He would face up to 20 years in prison if convicted on all counts.

“Providing humanitarian aid is not a crime. We urge the U.S. authorities to immediately drop all charges against Scott Warren,” the UN experts, including those dealing with migrant rights and the right to health, said in a joint statement.

Warren is a volunteer for No More Deaths, a charity that provides water, food and medical aid to mostly Central American migrants crossing Arizona’s southern deserts, where temperatures drop below freezing in winter and exceed 46 C in summer. Over 3,000 migrants have died trying to cross the area since 2001, according to Pima County data.

His arrest in January 2018 came hours after the group released a report that “documented the implication of Border Control agents in the systematic destruction of humanitarian supplies, including water stores, and denounced a pattern of harassment, intimidation and surveillance against humanitarian aid workers,” the UN experts said in their statement.

“The vital and legitimate humanitarian work of Scott Warren and No More Deaths upholds the right to life and prevents the deaths of migrants and asylum seekers at the U.S.-Mexican border.”

Defying increasing criticism from even some within his own party, U.S. President Donald Trump said on Tuesday he would likely go ahead with new tariffs on imports from Mexico to pressure it to clamp down on rising numbers of migrants entering the United States.