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St Elijah's Monastery stood as a place of worship for 1,400 years, including most recently for US troops.

In earlier millennia, generations of monks tucked candles in the niches, prayed in the chapel, worshipped at the altar.

The Greek letters chi and rho, representing the first two letters of Christ's name, were carved near the entrance.

This month, at the request of AP, satellite imagery firm DigitalGlobe tasked a high resolution camera to grab photos of the site, and then pulled earlier images of the same spot.

Before it was razed, a partially restored, 27,000-square-foot stone and mortar building stood fortress-like on a hill above Mosul.

Although the roof was largely missing, it had 26 distinctive rooms, including a sanctuary and chapel.

One month later, photos show "that the stone walls have been literally pulverised", said imagery analyst Stephen Wood, CEO of Allsource Analysis, who pinpointed the destruction between August and September 2014.

"Bulldozers, heavy equipment, sledgehammers, possibly explosives turned those stone walls into this field of grey-white dust. They destroyed it completely," he said from his Colorado offices.

On the other side of the world, in his office in exile, in Erbil, northern Iraq, Catholic priest Reverend Paul Thabit Habib stared in disbelief at the before and after images.

Top: March 2011; bottom: September 2014. The 1,400-year-old St Elijah's monastery has been completely destroyed [AP/DigitalGlobe]

"Our Christian history in Mosul is being barbarically levelled," said the 39-year-old cleric. "We see it as an attempt to expel us from Iraq, eliminating and finishing our existence in this land."

The Islamic State group, which now controls large parts of Iraq and Syria, has killed thousands of civilians in the past two years.

Along the way, its fighters have destroyed whatever they consider contrary to their interpretation of Islam.

St Elijah's joins a growing list of more than 100 religious and historic sites looted and destroyed, including mosques, tombs, shrines and churches.

Ancient monuments in the cities of Nineveh, Palmyra and Hatra are in ruins.

Museums and libraries have been pillaged, books burned, artwork crushed or trafficked.

US troops and advisers had worked to protect and honour the monastery, a hopeful endeavour in a violent place and time.

"I would imagine that many people are feeling like, 'What were the last 10 years for if these guys can go in and destroy everything?" said US Army reserve Mary Prophit, who was deployed there in 2004 and again in 2009.

Built in 590, tragedy struck at St Elijah's in 1743, when as many as 150 monks who refused to convert to Islam were massacred by a Persian general.

In 2003 St Elijah's was shuddered again, this time after a wall was smashed by a tank turret blown off in battle.

Iraqi troops had already moved in, dumping garbage in the cistern.

The US Army's 101 Airborne Division later took control, reportedly painting over ancient murals and scrawling their division's "Screaming Eagle" on the walls.

Then a US military chaplain, recognising its significance, began a preservation initiative.

Roman Catholic Army chaplain Jeffrey Whorton, who celebrated mass on the monastery's altar, was grief-stricken at its loss.

"Why we treat each other like this is beyond me," he said. "Elijah the prophet must be weeping."