Iraqi Prime Minister Mustafa Kadhemi paid his respects on Wednesday (July 8th) to the family of slain scholar and government advisor Hisham al-Hashemi, pledging to "avenge" his death.

Al-Hashemi, 47, was a specialist in extremist 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 al-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.

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 do not 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 al-Hashemi's widow.

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

Threats by Iran-backed groups

Al-Hashemi was a renowned researcher on the "Islamic State of Iraq and Syria" (ISIS) 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 al-Hashemi's killing would usher in a dark era in which prominent voices critical of political parties and armed groups would be violently silenced.

US Secretary of State Mike Pompeo on Wednesday demanded justice over the killing of al-Hashemi and highlighted threats against him by Iran-linked groups.

"In the days leading up to his death he was repeatedly threatened by Iran-backed armed groups," Pompeo told a news conference in Washington, without explicitly blaming Tehran.

"The US joins partner nations in strongly condemning his assassination and call(ing) for the government of Iraq to bring to justice the perpetrators of this terrible crime... swiftly," he said.