Iran has hanged up to 20 Sunni Muslim prisoners it says were involved in armed attacks.
State media said the group was executed on Tuesday, and was responsible for killing women and children between 2009 and 2011.
Iran, which is predominantly Shia, executed at least 977 people last year, according to Amnesty International.
Rights groups have criticised the latest executions, saying the trials were unfair.
The US-based group, International Campaign for Human Rights in Iran, said one of the men hanged, Shahram Ahmadi, claimed that the entire case against him was built on a forced confession.
None of the men's families were allowed to visit them before they were hanged, the organisation said.
Prosecutor-general Mohammad Javad Montazeri told Iranian television the men came from Kurdish areas and belonged to the extremist Tawhid and Jihad group.
Last year, Amnesty International said there had been a huge spike in the number of people executed in Iran, adding that courts were often "completely lacking in independence and impartiality".
The organisation said Iran was the second most prolific executioner in the world last year, after China, which executed more than 1,000 people.
After Iran, Pakistan executed 326 people, and Saudi Arabia 158 people, Amnesty International said.