A militia group in Iraq has purportedly used Instagram to put to a vote the fate of a captured Islamic State fighterwhich, if genuine, is “unquestionably” a war crime.

The account @iraqiswat, claiming to be that of Iraq Special Operation Force, posted an image to its 80,400 followers on Monday that appeared to show a captured Isis fighter.

According to the caption, the Isis jihadi had been arrested south of Mosul, and followers had one hour in which to decide whether he was killed or released.

“You can vote For (kill him or let him go) You have one houer to vote We will post his fate after one houer Tag your friends and take your right take your reveng from isis right now. Please we dont have the time just one houer so tag your friends,” the post reads.

A follow-up image indicated the “vote” had resulted in the fighter’s death.

But Phillip Smyth, a University of Maryland researcher into Shia armed groups, questioned the veracity of the “vote”, saying Monday’s post was not the first time the page had claimed to allow commenters to decide a prisoner’s fate.

“Many times this account has taken other photos of ghastly deeds done by militias or possibly by Iraqi government forces and then rebranded them, claiming to run some vote based on comments,” he said.

“It is very hard to double check and see if they are telling the truth that ‘people voted about how to deal with a prisoner’.”

The page, which is not official, regularly posts footage and pictures apparently taken by Iraqi government soldiers and Iranian-backed Shia militia fighting in the country.

“Most of what they put up is just straight propaganda, though that doesn’t mean the photos and brutality being shown is not genuine,” Smyth said.

The militiamen pictured had “no identifiable insignia, flags or equipment that would show them belonging to one force or another”, he added.

Instagram deleted the posts for violating its policy, but screenshots were tweeted by journalist Murad Gazdiev on Tuesday morning.

Subsequent images posted by the account celebrated online news coverage of the poll (“Google going crazy with our page”) and asked followers to “please tag your friends for more live revenge vote video killing isis with no mercy”.

Another confirmed that the original posts had been deleted by Instagram, and greeted “all the new followers”.

The account remains active because most of its 754 posts do not violate Instagram’s community standards. The sample applies to the apparently associated @iraqiswat2 account.

A spokesman for Instagram said serious threats of harm to public and personal safety were forbidden on the platform. “Some of the content on this account violated our policies, which is why we removed it as soon as it was reported to us.”

He was unable to confirm or comment on the account’s origins or authenticity.

Gideon Boas, who served as senior legal officer at the International Criminal Tribunal for the former Yugoslavia, said the images “unquestionably” appeared to depict a war crime.

“Whether or not that person is a combatant – and therefore receives the protection of prisoner of war status – or falls to be dealt with as a domestic criminal, either way, that person has to be subject to a criminal process,” he said.

Shia militia groups have played a key role wresting control of cities such as Tikrit and Bayji from Islamic State, and are expected to play a part in any future operations in Mosul, Isis’s Iraqi stronghold.

Smyth said the Iran-backed groups were “generally the most powerful in the country and operate as a parallel security apparatus to the Iraqi government, despite falling under their nominal rule”.

The groups, who have been accused of possible war crimes in the past, are in many cases more closely linked to the government in Tehran than Baghdad, Smyth said.