ISLAMABAD (AP) — A top U.N. agency is demanding justice for Pakistani social media activist and blogger Mohammad Bilal Khan, killed in a knife attack in Islamabad earlier this month.

Khan was an outspoken critic of the country's political and military leaders. The military has said state security forces were not responsible for his death.

UNESCO's director-general, Audrey Azoula, issued a statement on Wednesday, condemning the June 16 slaying and asking Pakistan to investigate the killing and bring the perpetrators to trial.

Azoula says "freedom of expression is a fundamental human right that must be preserved for everybody."

No group has so far claimed responsibility for the killing. Police say they're trying to trace a phone call, which they say Khan's assailants used to lure him to the area where he was killed.