AL-BAB, Syria (Reuters) - Syrian rebels who drove Islamic State from the town of al-Bab in northwest Syria this year discovered an extensive network of tunnels dug by militants as part of their defenses, a tactic that has slowed the military campaign against them.

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“The tunnels complicated the fighting a lot and stopped our advance for weeks,” said Mohammed Abu Yousef, a rebel in a group fighting under the banner of the Free Syrian Army as part of a Turkey-backed military campaign in north Syria.

FSA rebels in al-Bab said they had found about 15 km (9 miles) of tunnels under the town that had linked its central areas and jihadist headquarter buildings with the town’s fringes and battle fronts.

Islamic State has been steadily forced from much of its Syrian territory since late 2015. It has lost all its land along the border with Turkey as well as the desert city of Palmyra as it is repelled into its strongholds along the Euphrates basin.

It is under assault from three rival forces: FSA rebels backed by Turkey, Syria’s army supported by Russia, Iran and Shi’ite militias, and the Syrian Democratic Forces, an umbrella for Kurdish-led groups supported by a U.S.-led coalition.

A spokesman for the U.S.-led coalition against Islamic State described the militants’ use of tunnels in several different cities as “a challenge for our partner forces” and meant to allow them to “move undetected”.

In one tunnel, a little over a meter (yard) wide and high enough to allow a man to stand upright, the walls and ceiling were covered with chicken wire and an electrical cable ran above with light bulbs occasionally dangling down.

AMBUSH

The fighting to take al-Bab lasted for weeks in January and February, costing many lives as Turkish jets and armor pummeled Islamic State positions in the town and FSA groups tried to capture ground.

Evidence of the battle can be seen in its rubble-strewn streets. In one district, houses were partially collapsed from fighting and bombardment and the large aluminum water tank from a building’s roof lay on its side, dotted with bullet holes.

“When we entered an area and ensured it was clear of Daesh fighters they would suddenly appear behind us using the tunnels and they killed a lot of our people by outflanking them this way,” said Abu Yousef, using an acronym for Islamic State.

His fellow rebels infiltrated some of the tunnels themselves to ambush the jihadists and blew up others to prevent them being used, he said.

Some of the tunnels came out inside buildings in the town, including one that residents told rebels had previously been used by Islamic State for a prison.

Abu Yousef said residents told rebels that the tunnels were dug using pneumatic drills over a period of months. Inside some tunnels were ventilation shafts and rest points with mattresses and bedding.

At a high point overlooking al Bab, a narrow gash in the ground revealed the sloped opening to a tunnel near pitted concrete pillars of a damaged building from which hung slabs of roof, with twisted steel rebars poking out from the sides.