A member of the federal police is seen inside a tunnel used by Islamic State militants, on the outskirts of Albu Saif, which was recently retaken by Iraqi military forces, south of Mosul, Iraq February 22, 2017. REUTERS/Zohra Bensemra - RTSZSD4

A tunnel used by Islamic State militants is seen on the outskirts of Falluja, Iraq, May 28, 2016. REUTERS/Thaier Al-Sudani - RTX2ELXM

A tunnel used by Islamic State militants is seen in the town of Sinjar, Iraq December 1, 2015. REUTERS/Ari Jalal - RTX25ALX

A member of the Iraqi security forces stands next to a tunnel used by Islamic State militants during battle with Islamic State militants in Talkeef district north of Mosul, Iraq, January 20, 2017. REUTERS/Khalid al Mousily - RTSWFXU

A fighter from the Iraqi Shi'ite Badr Organization holds his rifle in an underground tunnel built by Islamic State fighters on the outskirts of Falluja, Iraq, May 28, 2016. REUTERS/Thaier Al-Sudani - RTX2ELX2

A tunnel used by Islamic State militants is seen in the town of Sinjar, Iraq December 1, 2015. REUTERS/Ari Jalal - RTX25ALR

A tunnel used by Islamic State militants is seen in the town of Sinjar, Iraq December 1, 2015. REUTERS/Ari Jalal - RTX25AM4

Blankets are seen inside a tunnel used by Islamic State militants on the outskirts of Albu Saif, which was recently retaken by Iraqi military forces, south of Mosul, Iraq February 22, 2017. REUTERS/Zohra Bensemra - RTSZSP4

A member of the federal police helps his comrade to get out the tunnel used by Islamic State militants on the outskirts Albu Saif, which was recently retaken by Iraqi military forces, south of Mosul, Iraq February 22, 2017. REUTERS/Zohra Bensemra - RTSZSOE

An Iraqi soldier holds his rifle in an underground tunnel built by Islamic State fighters in a village of Har Bardun, Iraq, April 28, 2016. REUTERS/Goran Tomasevic TPX IMAGES OF THE DAY - RTX2C2YS

A fighter from the Iraqi Shi'ite Badr Organization holds his rifle as he look a tunnel built by Islamic State fighters on the outskirts of Falluja, Iraq, May 28, 2016. REUTERS/Thaier Al-Sudani - RTX2ELYC

Fighters from the Iraqi Shi'ite Badr Organization stand near a tunnel built by Islamic State fighters on the outskirts of Falluja, Iraq, May 28, 2016. REUTERS/Thaier Al-Sudani - RTX2ELYW

A tunnel used by Islamic State militants is seen in the town of Sinjar, Iraq December 1, 2015. REUTERS/Ari Jalal - RTX25AM1

A tunnel used by Islamic State militants is seen in the town of Sinjar, Iraq December 1, 2015. REUTERS/Ari Jalal - RTX25AM2

Iraq's Rapid Response forces peer down a tunnel dug by Islamic State militants in a house in Albu Saif, south of Mosul, Iraq February 21, 2017. REUTERS/Alaa Al-Marjani - RTSZL70

Iraqi soldiers look a tunnel build by Islamic State fighters in a building destroyed by an airstrike in a village of Mahana some 60 km south of Mosul, Iraq, April 28, 2016. REUTERS/Goran Tomasevic TPX IMAGES OF THE DAY - RTX2C2XN

Fighters from the Iraqi Shi'ite Badr Organization look at a tunnel built by Islamic State fighters on the outskirts of Falluja, Iraq, May 28, 2016. REUTERS/Thaier Al-Sudani - RTX2ELYU

A tunnel used by Islamic State militants is seen in the town of Sinjar, Iraq December 1, 2015. REUTERS/Ari Jalal - RTX25AM0

A member of the federal police is seen inside a tunnel used by Islamic State militants on the outskirts of Albu Saif, which was recently retaken by Iraqi military forces, south of Mosul, Iraq February 22, 2017. REUTERS/Zohra Bensemra - RTSZSP3

A fighter from the Iraqi Shi'ite Badr Organization holds his rifle in an underground tunnel built by Islamic State fighters on the outskirts of Falluja, Iraq, May 28, 2016. REUTERS/Thaier Al-Sudani - RTX2ELXN

A member of the Peshmerga forces inspects a tunnel used by Islamic State militants in the town of Sinjar, December 1, 2015. REUTERS/Ari Jalal - RTX25ALY