The United Nations says it is "horrified" by the death of a juvenile offender in Iran, following reported beatings by prison guards after inmates protested to be released due to the coronavirus pandemic.

The Office of the UN High Commissioner for Human Rights (OHCHR) said on April 3 it had received information that Daniel Zeinolabedini died after he was reportedly "badly beaten" following a prison riot in Iran's West Azerbaijan Province on March 28.

Detainees at Mahabad prison were protesting against their "prison conditions and the failure of the authorities to temporarily release them amid the COVID-19 pandemic," spokesman Rupert Colville said in a virtual press briefing.

Media in Iran, which has been hit hard by the coronavirus pandemic, have reported unrest in several prisons, despite the temporary release of some 100,000 inmates to curb prison overcrowding.

There has been no report of the alleged incident in West Azerbaijan Province.

Zeinolabedini, who was on death row for a murder he allegedly committed in September 2017 at the age of 17, has claimed his innocence, according to the UN rights office.

He was put in solitary confinement and beaten by security officials at Mahabad prison, before being transferred to Miandoab prison in the same region, where he also suffered abuse.

"His family said he called them on March 31 to tell them he had been badly beaten, could hardly breathe, and desperately needed help. His death was confirmed on April 2," Colville said.

The rights office said it was "particularly shocked" because Zeinolabedini's conviction and death sentence were upheld by Iran's Supreme Court, despite international human rights law prohibiting death sentences for crimes committed by minors.

The spokesman called on the Iranian authorities to conduct an "independent and impartial" investigation into the death and hold those responsible to account.

Colville expressed concern about at the fate of "six other people who were also reportedly beaten during the riot on March 28 and taken to Miandoab prison," and urged the authorities "to take all measures to protect their lives."

With reporting by Reuters and AFP