Under the Iraqi town of Sinjar, Islamic State group militants built a network of tunnels, complete with sleeping quarters, wired with electricity and fortified with sandbags, the Associated Press reports.

There, they had boxes of U.S.-made ammunition, medicines and copies of the Quran stashed on shelves.

Thee tunnels were uncovered by Kurdish forces that took the city in northwestern Iraq earlier this month after more than a year of IS rule.

"We found between 30 and 40 tunnels inside Sinjar," said Shamo Eado, a commander from Sinjar from the Iraqi Kurdish fighters known as peshmerga. "It was like a network inside the city."

"Daesh dug these trenches in order to hide from airstrikes and have free movement underground as well as to store weapons and explosives," Eado said using an Arabic acronym for the Islamic State group. "This was their military arsenal."

The video, shot by a freelancer touring the town with Kurdish fighters, showed two tunnels running several hundred meters (yards), each starting and ending from houses, through holes knocked in walls or floors.

The narrow tunnels, carved in the rock apparently with jackhammers or other handheld equipment, are just tall enough for a man to stand in. Rows of sandbags line sections of the walls, electrical wires power fans and lights and metal braces reinforce the ceilings. One section of the tunnel resembled a bunker. Dusty copies of the Quran sit above piles of blankets and pillows. Prescription drugs — painkillers and antibiotics — lie scattered along the floor.

In another section of the tunnel, the footage shows stocks of ammunition, including American-made cartridges and bomb-making tools.

IS has been digging tunnels for protection and movement throughout the territory it controls in Iraq and Syria, even before the U.S.-led coalition launched its campaign of airstrikes against the group more than a year ago. "This has been part of ISIS' strategy from the very beginning," said Lina Khatib a senior research associate at the Arab Reform initiative, a Paris-based think-tank. "ISIS has been well prepared for this kind of intervention."

Eado, the peshmerga commander, said that as Kurdish forces clear Sinjar of explosives, he expects to find more tunnels and evidence of atrocities.