At least 80 people have been executed in Iran so far this year, marking a rise in its use of capital punishment, the UN's human rights office has said.

The majority of executions in Iran are by hanging and are handed down for drug-related offences, which fail to meet the threshold in international law for "most serious crimes", a category which covers acts such as murder, the AFP news agency reported.

"We are deeply concerned about the reported spike in executions in Iran since the beginning of this year," said Ravina Shamdasani, spokeswoman for the UN high commissioner for human rights.

"In just over seven weeks, at least 80 people have been executed. Some reliable sources indicate the figure could be as high as 95," she told reporters.

The case of two activists from Iran's Arab minority was of particular concern, Shamdasani said.

Hadi Rashedi and Hashem Shabani Amouri were reportedly executed in secret in January following proceedings that did not meet international fair trial and due process standards, she said.

After reportedly being denied access to lawyers and family members, and tortured into confessing, they were sentenced to death on what Shamdasani said were the "ill-defined" charges of "enmity against God" and "corruption on earth", as well as acts against national security.

"An escalation in executions, including of political prisoners and individuals belonging to ethnic minority groups, was notable in the second half of 2013," Shamdasani noted.

She said that at least 500 people are known to have been executed in 2013, including 57 in public, but that the number may be as high as 625.

The toll in 2012 was 314, according to Amnesty International.

The increased deployment of capital punishment dashed hopes after the "encouraging signs" last year when President Hassan Rouhani's release of a string of political prisoners.

Rouhani, a moderate, defeated a pool of conservatives in last June's presidential election.

"We regret that the new government has not changed its approach to the death penalty and continues to impose capital punishment for a wide range of offences," said Shamdasani.

"We urge the government to immediately halt executions and to institute a moratorium."

Source: Agencies