A specialist on Sunni extremism and a government adviser with a vast network of contacts among top decision makers, Hashemi was shot dead outside his Baghdad home in early July last year by gunmen on motorcycles.

The academic had also become outspoken against powerful Shiite armed actors aligned with Iran that Washington blames for rocket and other attacks against US interests and troops in Iraq.

"We promised to capture... (the) killers" of Hashemi, Kadhemi said on Twitter. "We fulfilled that promise," he added.

A security source told AFP that one of those arrested for the murder, Ahmed al-Kenani, was linked to Kataeb Hezbollah, a powerful pro-Iran faction that Hashemi criticised in his writings and media commentary.

Iraqi state television broadcast brief clips of the alleged confession of Kenani, a 36-year-old police lieutenant. Wearing a brown jumpsuit, he said he shot Hashemi with a pistol.

A security source told AFP that dozens of military tanks and counter-terrorism units were deployed Friday in Baghdad's heavily fortified Green Zone, where the US embassy -- a frequent target of rocket attacks -- is located.

- 'Ending impunity' -

Friday's announcement by the prime minister -- seen by pro-Iran groups as too close to Washington -- marks the first reported arrests over the murder.

Surveillance footage of the attack shown on state television purports to show Kenani carrying out the killing with three others, riding on two motorcycles.

Hashemi's support for popular protests that erupted in 2019 against a government seen by many as too close to Iran infuriated Tehran-backed Shiite factions in Iraq's Hashed al-Shaabi military network.

Earlier this month, dozens of people gathered in central Baghdad to remember him, holding pictures of the researcher and lighting candles.

The arrests represent "a positive step towards establishing accountability and ending impunity... and we hope that all perpetrators are held accountable," Ali al-Bayati, a member of the Iraqi government's human rights commission, said Friday.

But many doubt Kadhemi's ability to rein in armed factions.

The Hashed-al-Shaabi holds the second biggest bloc in Iraq's parliament and controls vast financial assets.

In a demonstration of its clout, it secured last month the release of one of its commanders, Qassem Muslah, after he was arrested on suspicion of ordering the killing of Ihab al-Wazni, a pro-democracy activist.

The judiciary said it had found "no proof" of Muslah's involvement in the murder.

- 'Don't care about spin' -

Killings, attempted murder and abductions have targeted more than 70 activists since a pro-democracy protest movement erupted against government corruption and incompetence in 2019.

Muslah's release was a blow to Kadhemi's efforts to win over the protest movement, and the prime minister has also been seen as powerless to stop attacks against US interests.

US forces, who have 2,500 troops deployed in Iraq as part of an international anti-Islamic State group coalition, have been targeted almost 50 times this year in the country.

The US launched air strikes against groups including Kataeb Hezbollah in February and June, hitting camps it allegedly uses borderlands between Syria and Iraq, in retaliation.

On Friday Kadhemi tried to assuage doubts over his government's ability to hold rogue actors to account.

"We have arrested hundreds of criminals -- murderers of innocent Iraqis," said Kadhemi, who is scheduled to visit Washington later this month.

"We don't care about media spin: we carry out our duties in the service of our people and in pursuit of justice," he added.

An Amnesty International researcher meanwhile called for the investigation into Hashemi's murder to extend to the highest levels of responsibility.

"A TV confession...is not a substitute for a proper trial based on solid evidence of who ordered the killing -- not just who pulled the trigger," Donatella Rovera said on Twitter.