The UN has issued an urgent call for Saudi Arabia to halt the execution of a young man who faces imminent beheading and crucifixion for crimes he reportedly committed as a child.

A Saudi court has upheld the sentence of Ali Mohammed al-Nimr, the son of a prominent government dissident, despite growing and high-level international condemnation.

Mr al-Nimr, who was arrested in 2012 for his participation in Arab Spring protests when he was just 16 or 17 years old, could now be put to death at any time.

The young man’s case has been the subject of fervent campaigning from rights groups including Amnesty International and Reprieve, who say he was tortured and forced to sign a false confession before being sentenced to “death by crucifixion”.

Now, a group of UN human rights experts have penned a joint statement calling on Saudi Arabia to “immediately halt the scheduled execution” and give Mr al-Nimr “a fair retrial”.

The experts, including the UN’s special rapporteur on extrajudicial or arbitrary executions Christof Heyns and Benyam Mezmur, the chair of the UN Committee on the Rights of the Child, said imposing the death penalty on someone who was a child at the time of offending and after allegations of torture was “incompatible with Saudi Arabia’s international obligations”.

“International law, accepted as binding by Saudi Arabia, provides that capital punishment may only be imposed following trials that comply with the most stringent requirements of fair trial and due process, or could otherwise be considered an arbitrary execution,” they said.

“In light of reports that the trial against Mr al-Nimr fell short of such standards, we call upon the Saudi authorities to ensure a fair retrial of Ali Mohammed al-Nimr, and to immediately halt the scheduled execution,” the experts added.

The French government has also taken the unusual step of adding its voice to calls for a stay of Mr al-Nimr’s execution. Foreign ministry spokesman Romain Nadal said: “France is concerned about the situation of Ali Mohammed al-Nimr, who was sentenced to death even though he was a minor at the time of the events.

Where most executions were carried out in 2014 Show all 10 1 /10 Where most executions were carried out in 2014 Where most executions were carried out in 2014 1. China (1000+) Getty Where most executions were carried out in 2014 2. Iran (289+) Getty Where most executions were carried out in 2014 3. Saudi Arabia (90+) Getty Where most executions were carried out in 2014 4. Iraq (61+) Getty Where most executions were carried out in 2014 5. USA (35+) Getty Where most executions were carried out in 2014 6. Sudan (23+) Getty Where most executions were carried out in 2014 7. Yemen (22+) Getty Where most executions were carried out in 2014 8. Egypt (15+) Getty Where most executions were carried out in 2014 9. Somalia (14+) Getty Where most executions were carried out in 2014 10. Jordan (11+) Getty

“Opposed to the death penalty in all cases and circumstances, we call for the execution to be called off.”

A court rejected Mr al-Nimr’s final appeal this month at around the same time as Saudi Arabia was chosen to head up a key UN panel on human rights.

But while the decision sparked outrage from campaigners and those affected by oppressive Saudi practices, the US State Department said it “welcomed” the move.

Speaking to AP’s Matt Lee, department spokesman Mark Toner said: “Frankly, we would welcome it. We’re close allies… We have a strong dialogue, a partnership with Saudi Arabia that spans many issues.