United Nations: Iran's authoritarian regime has been secretly executing hundreds of prisoners, according to a new UN report detailing growing rights abuses in the Islamic republic.

The mysterious executions at Vakilabad prison in Mashhad in eastern Iran were highlighted in a report compiled by Ahmed Shaheed, the new UN special rapporteur on the human rights situation in Iran.

Shaheed, who assumed responsibility for the mandate on August 1, billed this as an interim report cataloguing the most recent trends in the human rights situation in Iran.

The report details a raft of abuses from freedom of expression to the denial of women's rights to the torture of detainees, but perhaps the most shocking data was on the skyrocketing rate of executions.

The report said 200 officially announced executions had taken place in 2011 with at least 83, including those of three political prisoners, in January alone.

"Furthermore, authorities reportedly conducted more than 300 secret executions at Vakilabad prison in 2010," the report said.

"Vakilabad officials, in violation of Iranian law, allegedly carried out the executions without the knowledge or presence of the inmates' lawyers or families and without prior notification to those executed," it said.

"It has also been reported that at least 146 secret executions have taken place to date in 2011."

Shaheed also noted that four per cent of executions stipulated no charges, 100 juveniles on death row, and more than 100 executions this year alone for drug-related offenses.

Human Rights Watch counted 388 executions in Iran in 2010, while Amnesty International put the figure at 252, ranking the Islamic republic second only to China in the number of people put to death last year.

Tehran says the death penalty is essential to maintain law and order, and that it is applied only after exhaustive judicial proceedings.

Murder, rape, armed robbery, drug trafficking and adultery are among the crimes punishable by death in Iran.