The tunnels were found in the villages of Bartella and Bazaia, roughly four miles east of Mosul.
US Defence Secretary Ash Carter said the operation would follow recent military action in Mosul, Iraq.
Elite Iraqi forces are clearing areas retaken from Isis since the beginning of Mosul offensive
One refugee claimed families were thrown out of their tents while they were still in them.
Daesh militants launched a second counter-offensive since operation to recapture Mosul began.
Iraqi forces scour derelict buildings for Isis fighters still hiding out in the towns.
US soldiers are wearing masks to protect against sulphur fumes.
The Arabic 'meem' symbol spray-painted on Mosul's walls is the most visible sign of resistance to Isis rule.
From intelligence gathering to disrupting online propaganda - will hacking help?
Defiant Alghad Radio connects the people trapped inside Mosul with the outside world.
Iraqi PM said operations going quickly as Kurdish troops and the Iraqi army advance into Mosul.
Secretary of Defence Michael Fallon said the military was launching the online attacks against IS.
Khaleel Aldakhi and wife Ameena Saeed Hasan risk their lives to save distressed girls enslaved by Isis.
Residents of Sheikh Amir village near Mosul said Isis militants have destroyed all houses belonging to Shias.
It is believed around 5,000 IS fighters in Mosul are being used as suicide bombers to halt advances.
Lieutenant General Talib Shaghati told reporters he was calling on militant group to surrender its positions.
Landmines and booby traps suggest the advance into Mosul will be slow and dangerous.
In the last 10 days about 5,000 refugees have arrived in Syria from Mosul.
Experts have warned Isis may use mustard gas as a last resort as aid organisations order gas masks.
The images show two children – who appear no older than 13 – standing over prisoners with handguns.
IBTimes UK reports from northern Iraq as Peshmerga advance column probes Islamic State's last bastion.
Iraqi and Kurdish forces retake 200sq km area on first day of Mosul battle.