Hashemi, 47, was a specialist in jihadist movements and had developed a vast network of top decision makers, armed groups and rival parties, often mediating among them.

He was shot dead outside his Baghdad home on Monday night by gunmen on motorcycles, leaving behind a wife, three sons and a daughter.

On Wednesday, Kadhemi paid his respects to the family, calling Hashemi -- a personal friend and advisor -- a "hero".

"Those afraid of a word can only be described as cowards. Hisham did nothing but try to help Iraqis through his words," said Kadhemi, hugging the deceased's tearful three sons Issa, Moussa and Ahmed.

Their names translate in Arabic to Jesus, Moses and another name for the Prophet Mohammed.

The three boys had rushed outside their home on Monday after hearing gunshots and helped neighbours pull their father's bullet-riddled body from his car.

"This behaviour is not Iraqi. Iraqis don't kill Iraqis," Kadhemi said.

"I will avenge him, and God willing his killers will not go free. I am your brother, and Issa, Moussa and Ahmed are my children," the premier told Hashemi's widow.

"This is my duty and the state's duty," he added.

Hashemi was a renowned researcher on the Islamic State group and had more recently become outspoken against rogue armed actors in Iraq.

He was no stranger to intimidation efforts, but those close to him told AFP he had received more serious threats from Iran-backed groups in recent weeks.

Experts have voiced fear that Hashemi's killing would usher in a dark era in which prominent voices critical of political parties and armed groups would be violently silenced.

Already, there has been no accountability for more than 550 people killed in protest-related violence since October, when mass rallies slammed Iraq's government as corrupt, inept and beholden to neighbouring Iran.

Among them are around two dozen activists who were shot dead, often by masked assailants on motorcycles.