The elite Special Air Service (SAS) troops of the British Army reportedly found at least 50 severed heads of sex slaves murdered by Islamic State (ISIS) militants as they reached the last stronghold of the terror group in Syria.

ISIS militants reportedly beheaded dozens of Yazidi women before dumping their heads in dustbins, according to Daily Mail. The discovery was made after special forces entered the besieged town of Baghuz on the banks of the Euphrates in Eastern Syria, which is the last remaining stronghold of the Islamist caliphate.

Reports state that the SAS soldiers fired 600 mortar bombs and tens of thousands of machine-gun rounds earlier this month, forcing the terror group into a network of tunnels beneath the town. Over 100 jihadis were killed during the battle, while two British soldiers were reported wounded with non-life-threatening injuries.

A member of the Turkish-backed Free Syrian Army (FSA) stands guard in front of a ISIS flag in the border town of Jarablus, August 31, 2016, Syria. Turkish troops and Turkey-backed rebels have been fighting Kurdish-led forces and IS since Turkey's incursion into Syria on Aug. 24. (Getty Images)

"In their hour of defeat, the jihadis' cruelty knew no bounds. They conducted a cowardly slaughter of these desperately unfortunate women as a final act of depravity and left their severed heads behind for us to find," a source told the outlet on Sunday. "The motivation for such a sickening act is beyond comprehension for any remotely normal human being."

"None of the SAS troops who entered Baghuz will forget what they saw, which some soldiers likened it to a scene from the film 'Apocalypse Now'. Their only solace is that they have contributed to bringing the Islamic State's reign of terror to an end," the source added.

The terror group, at its prime nearly five years ago, controlled nearly 34,000 square miles of territory stretching from Western Syria to Eastern Iraq as they proclaimed their new "caliphate" and imposed a barbaric rule on nearly eight million people in the region. The group also generated billions of dollars through robbery, kidnapping, extortion and also from the oil in the region.

However, after years of battle between the fighters and coalitions formed by multiple nations, the terror group now consists of nearly 200 militants as of Saturday night, who have been confined to one-fifth of a square mile of scrubland near Baghuz called Hawi al-Dandal. The militants have reportedly held an equal number of civilians captive in the region.

Iraqi counter terrorism soldiers relax during a lull in fighting in the offensive to recapture the city of Mosul from Islamic State militants, on October 23, 2016, near Bartella, Iraq. (Getty Images)

"The battle proper began on February 9. In the first two days, 37 IS fighters were killed and 19 enemy positions were destroyed, including the jihadis' operational control center in a mosque in Baghuz on February 11," a source told the Mail.

"An advance by the SAS and SDF troops caused IS to go underground, utilizing a network of tunnels under the town, but the rats couldn't escape because, even with the heavy cloud cover and dust storms, we were able to use drones effectively and identify openings to these tunnels. Co-ordinates for these positions were passed to SAS mortar teams and the US-French artillery units which pounded these positions."

"There was then a lull in the fighting while a humanitarian corridor was established, allowing around 1,500 civilians to be driven out of Baghuz in a column of 17 trucks. Most of those leaving the town were women and children, but the convoy also included IS fighters disguised as women, who were taken into captivity," the source added.