Iran executed nearly 1,000 prisoners last year, the highest number in two decades, and hundreds of journalists, activists and opposition figures languish in custody, a United Nations investigator said Thursday.

Ahmed Shaheed, U.N. special rapporteur on human rights in Iran, voiced particular concern about executions for crimes committed by children under 18. This was "strictly and unequivocally prohibited under international law.

There had been a "staggering surge in the execution of at least 966 prisoners last year — the highest rate in over two decades," Shaheed told a news briefing.