The video, believed to have been captured in the terror group’s last holdout in Baghouz, near the Iraqi border, shows a guided weapon thudding into buildings.
It is thought to show a recent confrontation between the extremists and Syrian Democratic Forces, a Kurdish-led coalition backed by Washington.
The SDF has launched a final push this week to expel ISIS fanatics from the enclave.
Thousands of people have fled Baghouz as the fighting rages, including women and children but also suspected jihadists.
Today the Kurdish-led SDF were advancing slowly towards hundreds of jihadists, with both sides locked in heavy clashes.
Footage shared online purportedly showed an ISIS fighter launching an anti-tank guided missile which spirals through the air before blowing up the top floor of a building.
In another shot, a rocket destroys a wall with a missile, while a man shouts ‘Allahu Akbar’ – ‘God is great’.
The Syrian Observatory for Human Rights said the US-backed fighters were making painstaking progress with mines scattered throughout the sector.
Hundreds of people fled the IS holdout in the night of Tuesday to Wednesday, an SDF spokesman said.
The SDF believes 400 to 600 jihadists may be holed up there, including foreigners and other hardened militants.
Last defence: Dramatic footage has emerged purportedly showing ISIS fanatics firing anti-tank missiles at US-backed forces in a bloody defence of their last Syrian stronghold
Missile strike: A missile destroys part of the wall of a compound. The footage is thought to show the terror group’s last stand in Baghouz
Men who had escaped were patted down and taken for retina scans.
Since early December, more than 38,000 people, mostly wives and children of IS fighters, have fled into SDF-held areas, the Observatory said.
That figure includes around 3,400 suspected jihadists detained by the SDF, according to the monitor.
‘The bombing was unimaginable, we ran from one place to the other,’ said Hala Hassan, 29, a woman from Syria’s Deir al-Zor who escaped with her five children.
She said ‘fighters from all nationalities’ were in the enclave.
‘There was no food. We ate grass from the ground like sheep… ISIS had blocked the roads and smugglers wanted thousands of dollars,’ she said.
The SDF launched a military offensive in September to expel ISIS from the oil-rich province of Deir Ezzor.
Wreckage: A Syrian Democratic Forces (SDF) armoured vehicle drives through destroyed streets near the front line in Baghouz
A diagram showing the last remaining ISIS territory in Syria in the eastern province of Deir Ezzor, where U.S.-backed coalition forces have launched a bid to push them out
The Kurdish-led alliance has pressed the group into a patch of less than one square mile.
Since then, more than 1,300 SDF fighters and 650 jihadists have been killed, while more than 400 civilians have also lost their lives.
US President Donald Trump on Monday said the coalition may declare victory over IS in Syria within days.
A victory in Baghouz would allow the United States to withdraw all its 2,000 troops from Syria, as announced by Trump in December.
But a top U.S. general said last week the militants would be an enduring menace after the withdrawal, as they retained leaders, fighters, and resources that would fuel further insurgency.
Smoke from an airstrike is seen on the front line in Baghouz as the Syrian Democratic Forces (SDF) stepped up their final campaign to oust the remaining ISIS fighters
Fighters: Syrian Democratic Forces occupy a building near the front line as they try to seize the last remaining pocket of ISIS territory