ISIS has released a significant new execution video from the historic city of Palmyra, in the Syrian desert.

Child executioners are shown in the video being forced to brutally slaughter a group of more than 25 regime soldiers.

The video shows the soldiers lined up on their knees on the stage of the Roman amphitheatre, which had formerly been used for an annual festival in the city.

A baying crowd of men and boys gathered in the restored ruin waiting for the slaughter, many wearing military uniforms and headscarves.

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Harrowing footage: Child executions were forced to brutally slaughter a group of more than 25 regime soldiers in Palmyra

Soldiers: The child executioners were paraded into the amphitheatre when the condemned regime soldiers had been lined up on the stage on their knees

Stage set: ISIS lined the condemned regime soldiers up on the stage of the ancient amphitheatre, which was formely used for an annual festival in the Syrian city

Behind each condemned man stands a child executioner, all wearing desert camouflage and brown bandanas, many of whom appear to be no older than 12 or 13 years old.

ISIS fighters can be seen standing or sitting on top of the ancient ruins, watching over the crowd as they wait for the slaughter.

A leader speaks to the audience, in front of an enormous ISIS flag draped across the back of the stage.

Waiting crowd: The men kneeled along the stage with a giant ISIS flag hanging in the background

Condemned: The video shows the men being led into the amphitheatre, and includes close ups of each of their faces

ISIS take-over: Palmyra, which was captured by ISIS at the end of May, lies in the middle of the Syrian desert

On a command, the condemned soldiers are killed simultaneously with a shot to the head.

ISIS captured the historic city on May 21, and has been systematically destroying the city's treasured ancient monuments.

The group was accused of executing hundreds of people in and around Palmyra since it swept into the city in May, after a lightning advance across the desert from its stronghold in the Euphrates Valley to the east.

ISIS is known for using children – dubbed ‘caliphate cubs’ – as suicide bombers, soldiers and executioners in their attempts to instill fear across Iraq and Syria.

Authorities fear that the use of children as soldiers and suicide bombers will only increase in the coming months.

Militants have kidnapped more than 500 children in recent weeks, and Iraqi police chiefs fear the children will be brainwashed into joining the ISIS ranks.

The children were snatched from Iraq’s Anbar and Diyala provinces and taken to the group’s bases, so they can be trained for use in ‘terror attacks’, according to Iraqi authorities.

This is not the first such execution in the Roman amphitheatre in the ancient Syrian city.

Executions being carried out in the Palmyra amphitheatre were first reported on May 27 by the Syrian Observatory for Human Rights, a Britain-based monitor, less than a week after ISIS captured the city.

The extremists 'called people to watch' the executions of 20 local men in the amphitheatre at the end of May.

The murdered locals were accused of being government supporters, according to a report from the Syrian Observatory for Human Rights.

Line-up: The child executioners - many of whom appear to be no older than 12 or 13 - are paraded in the video in front of the condemned regime soldiers

Blood-thirsty: A baying crowd of men and boys gathered in the amphitheatre to watch the 'spectacle'

Support: Many wore military uniforms and headscarves while waving flags in support of ISIS

'ISIS executed 20 men by firing on them in front of a crowd gathered in Palmyra's Roman theatre, after accusing them of fighting for the Syrian regime,' Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP at the time of the executions.

He added: 'ISIS gathered a lot of people there on purpose, to show their force on the ground.'

ISIS reportedly carried out more than 200 executions, including of civilians, in and around Palmyra in the period when it captured the city.

At the time, Syria's antiquities director Mamoun Abdelkarim said he feared the killings could signal the start of 'the group's barbarism and savagery against the ancient monuments of Palmyra'.

'Using the Roman theatre to execute people proves that these people are against humanity,' he told AFP.

The Greco-Roman ruins at Palmyra are listed as a UNESCO World Heritage Site, and the city's capture by ISIS prompted international concerns for the fate of its spectacular ancient treasures.

ISIS has regularly released videos of its mass executions, with slick production and gruesome violence that experts say is a key propaganda tool for the group.

UNESCO has condemned the jihadists' destruction of antiquities in the Syrian city of Palmyra, describing it as an attempt to strip the people of their heritage in order 'to enslave them'.

Adult fighters: The condemned men were led into the stadium by gun-wielding ISIS fighters, but many of the executioners appear to be teenagers

Expectant: A blood-thirsty crowd of men and boys gathered to watch the brutal executions, while watched over by ISIS fighters sitting and standing atop the restored ruins

'These new destructions of cultural goods of the site of Palmyra reflect the brutality and ignorance of extremist groups and their disregard of local communities and of the Syrian people,' said Irina Bokova, UNESCO director-general.

Among the antiquities lost so far is the Lion Statue of Athena, destroyed at the end of May, which was more than three-metres high.

Also known as the Lion of Al-Lat, it dated back to the first century AD and is one of the most significant treasures to have been destroyed within the city.

The destruction of the lion came just days after ISIS gathered Palmyra’s citizens together and publicly promised not to harm the city’s ancient buildings.

But the fighters insisted that they would ‘pulverise’ any statues they discovered in the city that they believed citizens secretly prayed to.

The limestone statue was discovered in 1977 by a Polish archeological mission at the temple of Al-Lat, a pre-Islamic Arabian goddess.

'The destruction of funerary busts of Palmyra in a public square, in front of crowds and children asked to witness the looting of their heritage is especially perverse,' she said.

'These busts embody the values of human empathy, intelligence and honour the dead... Their destruction is a new attempt to break the bonds between people and their history, to deprive them of their cultural roots in order to better enslave them,' she added.

She urged action against the 'manipulation of religion'.

ISIS's harsh version of Islam considers statues and grave markers to be idolatrous, and the group has destroyed antiquities and heritage sites in territory under its control in Syria and Iraq.