The Pentagon has confirmed U.S. Special Forces are on the ground in Iraq, taking part in the battle to retake Mosul, Iraq’s second largest city, which fell to ISIS two years ago. Despite President Obama’s pledge against having boots on the ground, the Pentagon said U.S. Special Forces are on the front lines and finding targets for U.S. airstrikes in an effort to ensure more accurate targeting of ISIS strongholds in the city. The offensive to take Mosul is expected to take weeks, if not months.

Iraqi forces reportedly captured 20 ISIS-held villages on Monday. Aid groups fear the fighting will force over a million residents of Mosul to flee. Mosul, once home to more than 2 million people, fell to ISIS fighters more than two years ago. The operation to retake it is expected to be the most complex yet for Iraq’s military.

In a statement on Monday morning, the American general in charge of the coalition’s war in Iraq and Syria, Lt Gen Stephen Townsend, openly acknowledged the presence of “forward air controllers” amongst the US “advisory” contributions to the battle.

Those controllers – known as Joint Terminal Air Controllers, or Jtacs, and drawn from US special operations forces – are the troops on the ground who spot for airstrikes, in an attempt to ensure greater accuracy of aerial bombardment. Their presence indicates that US troops, while not formally in a combat role, are on the frontlines and willing to use substantial airpower in the battle.

The UK and France are also believed to have their own special forces involved in retaking Mosul. Earlier this month, two French special operations forces training peshmerga were seriously injured by a jury-rigged Isis drone which exploded after crashing near them in Dohuk, 46 miles north of the city.  While it is unclear what specifically the UK and French contributions to the battle are, it is believed British special forces will also aid in calling in airstrikes.

As the long-anticipated battle began on Monday, US warplanes launched substantial bombing runs on positions around Mosul. The US reported attacking three units of Isis fighters, two staging areas for their operations, 12 Isis rally points and even a bridge. Six tunnel entrances, an anti-artillery system, four generators powering communications-signal towers and four mortar systems were destroyed, according to the US military.

The large volume of forces needed for the Mosul fight – Iraqi and coalition officials say at least 30,000 will take part – has overcrowded the few bases that Iraq’s military has along the Mosul front line.  As Iraqi Army troops and their allies advance on the city of Mosul they have discovered an intricate network of underground tunnels all around the city.

The network of tunnels is believed to be used for supplies as well as a means for ISIS fighters to vanish underground and pop up elsewhere for attacks.  The tunnels are  ideal hiding places for machine gunners, grenade throwers and suicide bombers.