Iran has executed 40 people since the beginning of 2014, according to Amnesty International, with at least 33 carried out in the past week.
The human rights organisation released a statement detailing 21 executions confirmed by Iranian officials, and another 19 that were reported through "reliable sources".
"The spike in the number of executions carried out so far this month in Iran is alarming," said Hassiba Hadj Sahraoui, Amnesty International’s Deputy Director for the Middle East and North Africa.
Beginning on January 9, 2014, more executions were carried out in Iran than during the entire month last year, said the group.
The Iranian president, Hassan Rouhani, who decisively won the national election in June 2013, promised to follow a "path of moderation" in international affairs and to ease restrictions on civil liberties.
But from the time of his inauguration in August to the end of 2013, more than 300 people have been executed, according to a figure tallied by the Iran Human Rights Documentation Centre (IHRDC).
IHRDC also documents the charges of those executed since January 5 – the most being related to drug trafficking.
“In Iran drug-related offences are tried in Revolutionary Courts which routinely fell far short of international fair trial standards. The reality in Iran is that people are being ruthlessly sentenced to death after unfair trials, and this is unacceptable,” said Sahraoui.
"The Iranian authorities' attempts to change their international image are meaningless if at the same time executions continue to increase," he added.
In the past Tehran has said the death penalty is essential to maintain law and order, and that it is applied only after exhaustive judicial proceedings.