A band of Iraqi Christian militia known as the Babylon Brigades are helping to reclaim the area around Mosul from Islamic State (Isis/Daesh). These Christian fighters are driven by a desire to keep their community alive after Isis threatened to destroy it for good. Duraid Elias, their commander, told Reuters: "We are proving to the world that Christians are not weak. We are stronger than they imagined."

Elias said his forces were demolishing homes thought to belong to Islamic State fighters, to keep them from ever returning. "This is war," he said. "An eye for an eye and a tooth for a tooth."

The heavily armed men of the Babylon Brigades wear an assortment of military uniforms. Some sport black headbands declaring their devotion to Jesus or the Virgin Mary, or religious icons to their bulletproof vests. The bands of Christian fighters travel across the desert in pickup trucks bearing large wooden crosses.

The Babylon Brigades helped to reclaim a fourth century monastery near Mosul from Islamic State. The jihadists at the Mar Behnam monastery had burned a collection of books about Christian theology, scraped off inscriptions written in Syriac – the language apparently used by Jesus – and demolished sculptures of the Virgin Mary and the monastery's patron saint.

Islamic State graffiti covers the monastery's walls, including the group's motto: "Remaining and expanding". Another scrawl includes the date Dec. 24, 2014 – one of two Christmases the jihadists spent in control of the site. They removed the site's crosses and tried to erase any mention of Behnam, the son of an Assyrian king who, according to popular legend, built the monastery as penance for killing both his children after they converted to Christianity.

Elias told Reuters that the jihadists had converted Mar Behnam, Iraq's largest monastery, into a headquarters for the Hisba – a moral authority that enforced strict rules against such things as smoking, men shaving their beards and women baring their faces in public. A sitting room had been turned into a medical clinic, and the monks' bedrooms were used to hold transgressors. A remote corner of the complex was filled with dozens of satellite dishes the commander said had been confiscated from residents nearby.

"Their fundamental goal was to destroy Christian history and civilisation in the Nineveh plains," Elias said. The Nineveh plains, north and east of Mosul, are a mosaic of ethnic and religious communities with roots dating back to ancient Mesopotamia.

The Sunni Muslim hardliners of Isis have targeted the adherents and religious sites of those minority groups across the area, which it seized in 2014. At the time, the group issued an ultimatum to Christians: pay a tax, convert to Islam, or die by the sword. Most fled toward the autonomous Kurdish region, including a few dozen monks who left Mar Behnam with only the clothes on their backs.

The battle for Mosul, launched five weeks ago, is turning into the largest military campaign in more than a decade of conflict in Iraq since the 2003 US-led invasion that toppled Saddam Hussein.

The Iraqi military estimates around 5,000 Islamic State fighters are in Mosul. A 100,000-strong coalition of Iraqi government forces, Kurdish fighters and Shi'ite paramilitary units is surrounding the city. Mosul's capture would be a major step towards dismantling the caliphate, and Islamic State leader Abu Bakr al-Baghdadi, believed to have withdrawn to a remote area near the Syrian border, has told his fighters to stay and fight to the end.