"We promised to capture... (the) killers" of Hashemi, Kadhemi said on Twitter.

"We fulfilled that promise," he added in Arabic, English and Kurdish.

Hashemi, a specialist in militant networks with a vast network of contacts among top decision makers, was shot dead outside his Baghdad home in early July last year by gunmen on motorcycles.

Iraqi state TV broadcast brief clips of the apparent confession of one suspect, 36-year-old police lieutenant Ahmed al-Kenani.

Wearing a brown jumpsuit, Kenani confessed to shooting Hashemi dead using a pistol.

According to reported surveillance footage of the attack shown on state TV, Kenani carried out the killing with three others, riding on two motorcycles.

Friday's announcement marks the first reported arrests made over a murder that shocked the country, where killings of activists have surged over the last year.

- 'Positive step' -

"It is 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.

As well as being an expert on Sunni extremism in Iraq, Hashemi had become outspoken against powerful Shiite armed actors aligned with Iran.

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

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

Earlier this month, dozens of people gathered in central Baghdad to remember Hashemi, who was 47 when he was gunned down, holding pictures of the researcher and lighting candles.

He was described by one attendee as the "best security analyst Iraq has ever known".

An interior ministry official at that vigil promised that Hashemi's killers would be held to account.

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

A security source told AFP Friday that Kenani was linked to Kataeb Hezbollah, a pro-Iran Shiite armed factions in Iraq who Hashemi was critical of in his writings and media commentary.

Earlier this year, the US struck the Iranian-backed Iraqi paramilitary force, stationed along the Iraqi-Syrian border.

In a demonstration of its clout, the Hashed last month secured 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.

His release was a blow to Kadhemi's efforts to win over Iraq's pro-democracy protest movement, which has seen more than 70 activists targeted in assassinations or attempted assassinations since 2019.

But the prime minister on Friday attempted to assuage doubts over his government's ability to hold rogue actors to account.

"We have arrested hundreds of criminals -- murderers of innocent Iraqis," Kadhemi said, 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."