Islamic State fighter said men were from Tabqa air base, captured this week

Men in only their underwear form a long line stretching across the desert

Sickening footage appears to show Islamic State militants parading around 250 captured soldiers through the desert in their underwear before they are killed and their bodies piled on the bare earth.

An Islamic State fighter claimed the men were from the Syrian government's Tabqa air base which extremists seized on Sunday, potentially handing them warplanes, tanks, artillery and ammunition.

The video, which has not been independently verified, is too graphic to be published in full.

It begins by showing dozens of men being marched through the desert wearing only their underwear. It then fades to black, resuming with a pile of bloodied bodies stacked on top of one another.

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WARNING GRAPHIC CONTENT

'Another ISIS war crime': Footage and photographs have emerged of Islamic State fighters marching more than 200 soldiers across the desert to their deaths in only their underwear after capturing Syria's Tabqa air base

Horror: The video, too graphic to be published in full, fades to black before revealing a chain of men's bodies

Slaughter: A separate image showed masked gunmen preparing to shoot seven men from the same air base. The chilling photograph was released by the Raqqa Media Center yesterday and tallies with other reporting

As the horrific footage progresses it pans slowly across a vast line of men who appear to be dead, and whose bodies have been laid out one by one.

The line forms a slow crescent across the desert, seemingly stretching to the horizon as militants stand beside it. Eventually, after more than a minute, the cameraman reaches the end of the line.

The precise death toll is uncertain and other sources put it at lower than 250, but at least 150 bodies are visible in the shaky video.

Its description on Youtube said it showed the execution of Army officers and Nusayri people, a significant minority of Shia Muslims in Syria.

A caption to another version of the video said: ' The 250 shabeeha taken captive by the Islamic State from Tabqa in Raqqa have been executed.' Shabeeha is the Islamist name for soldiers loyal to President Bashar al-Assad.

An Islamic State fighter in Raqqa told Reuters: 'Yes we have executed them all'.

Marched: Earlier stills from the video showed men being paraded in their underwear and ordered to chant

Before they were killed, the men's captors chanted 'Islamic State' - to which they replied: 'It will remain'.

Islamic State won a week-long battle on Sunday to capture the Tabqa base, which is 25 miles from their Syrian stronghold in Raqqa.

Syria's authorities insisted at the time that their soldiers had 'successfully regrouped' - suggesting they were later hunted down by the militants and executed.

The captured men were escorted closely by Islamic State militants dressed in black and waving flags

Despite heavy losses on the Islamic State side, the base's capture prompted fresh fears that the fighters have got hold of advanced military technology which will allow them to cement their self-declared regime.

Video footage has already suggested Islamic State fighters have drones which were used to shoot reconnaissance footage of an army base.

Today's reported mass killing also underscores how the group uses images of violence as much as violence itself to terrorise its opponents, as it sweeps further into Syria and Iraq.

Nadim Houry, deputy director of Human Rights Watch for the Middle East, described the video as 'another ISIS war crime'.

And yesterday a UN commission accused the extremist group of committing crimes against humanity in Syria, similar to those it has already committed in Iraq.

Yet the extremists are also in touch with modern ideas of PR, releasing glossy magazines in English which feature mutilated bodies to promote their cause in the West.

The Home Secretary has sounded fresh warnings over the radicalisation of young Brits, with authorities fearing around 500 have joined an array of jihadi groups in Iraq and Syria.

Extremists have declared a self-styled caliphate, a Sunni regime ordering its subjects to operate under an extreme interpretation of Sharia law.

It has opened up three fronts in the fighting in Syria, which is already home to a bloody and long-running civil war between President Assad's forces and anti-government rebels.

Captured: A resident of Tabqa waves the Islamic State flag on Sunday after militants seized the nearby air base

Death march: Another video appearing to show the same march through the desert was put on social media

There were chants of 'Islamic State', to which the men replied 'It will remain', their hands behind their backs

Tormented: At one point the men appeared to be taunted by their captors as some turned towards the camera

Today CNN claimed the militants in Iraq have also been burning oil wells near the town of Zummar, which is crucial because it is near a road which links Mosul to the Syrian border.

The network suggested fighters are attempting to 'cover their tracks' as Kurdish Peshmerga fighters draw closer.

The Islamic State extremists now control roughly a third of Syria, mostly areas in the north and east of the country, as the U.S. threatens air strikes similar to those already used in Iraq.

But the situation is complicated as Washington has claimed the Syrian government - embroiled in a bloody civil war - is part of the problem despite offering itself as a force against extremism.

French President Francois Hollande added today that President Assad, whose forces used brutal force to crush what began as a peaceful uprising three years ago, was no ally in the fight against Islamic State.

Sprawling: An image of what is believed to be the air base, just south of the city of Tabqa, shows its long fence

Arsenal: Campaigners say the base contained weapons and jets which may now be used by the militants

UN PEACEKEEPERS CAPTURED Some 43 UN peacekeepers have been detained by gunmen in the Golan Heights, near Syria's border with Israel, where fighting has raged between rebels and government forces. The identity of the armed group is not known. Several rebel groups operate in the Golan, but the Islamic State group has no known presence there. The UN said another 81 peacekeepers are 'currently being restricted to their positions' as the Syrian government denounced the 'kidnapping'. In a statement issued by the Syrian Foreign Ministry, the government said it holds 'the terrorist groups and those who support them fully responsible for the safety of the UN peacekeepers, and calls for their immediate release.' The troops are part of UNDOF, the mission that has been monitoring a 1974 disengagement accord between Syria and Israel after their 1973 war. As of July, UNDOF had 1,223 troops. Advertisement

Today Syrian warplanes hit Islamic State targets in the eastern stronghold of Deir al-Zor and killed some of the group's commanders, according to the British-based Syrian Observatory for Human Rights.

The Observatory, run by a Syrian emigrant from his terraced house in Coventry, said the planes struck a building used as an Islamic State headquarters during a meeting of its commanders.

Syrian state TV reported that the army 'eliminated more than 10 terrorists' in an attack east of Deir al-Zor military airport, including two Islamic State leaders, and destroyed 14 armoured vehicles.

Another video posted online today appeared to show at least one Syrian soldier being interrogated before a group of other captured men in their underwear.

The soldier identifies himself as a Shia Muslim officer from the same sect as many high-ranking military officers and President Assad. Islamic State members are Sunni Muslims.

'How many have you killed? How many have you raped?' the interrogator shouts. The soldier replies: 'None. I've been stationed here in the airport.'