While some are motivated by a genuine desire to help, others have come seeking personal glory, fighters say.
Iraqi forces and Kurdish forces have recaptured four villages from Islamic State of Iraq and Levant (ISIL) group near its stronghold of Mosul, reports say.
The offensive started at 5:30am local time from various fronts and so far.
Iraqi forces managed to retake the villages of Tal Hamid, Qarqasha, Abzakh and Qura Takh.
Clashes are still ongoing in a fifth village, Sateeh.
The advance is part of a wider security operation to retake Iraq’s second largest city, which has been under the control of ISIL, also known as ISIS, since 2014.
ISIL fighters tried to slow down the offensive with two suicide car bombs near the village of Sateeh, but the Kurdish Peshmerga soldiers leading the offensive blew up the car before they reached them.
ISIL fighters are now burning tyres near their positions in an attempt to cover themselves from US-led coalition aircraft, Kurdish local television reported.
Peshmerga engineering teams are defusing bombs planted by ISIL in the four liberated villages.
The battle for Mosul, ISIL’s de facto capital in Iraq and the largest city anywhere in its self-proclaimed caliphate, is expected later this year, but plans have not been finalised, officials and diplomats in Baghdad have said.
Army, police and special forces are expected to participate, with air support from a US-led coalition.
Iraqi troops are currently securing villages around Qayyarah, an airbase which is being used to prepare for the final push towards Mosul.
More than 500 US troops will reportedly help Iraqi forces to transform the base into a staging area.
As ISIL is pushed out of places like Ramadi and Fallujah , sectarian tensions in Sunni-majority towns taken by Shia-led forces have resurfaced.
The forces trying to retake Mosul include the Kurdish regional government, fighters loyal to the pro-Sunni former provincial governor and the Shia-led central government.
Iran-backed Shia militias are also believed to be formalising their participation in northern Iraq.
Militias have been accused of torture and executions in recent military operations in Sunni areas and Iraq’s government is investigating the alleged abductions of more than a thousand Sunni men who are missing.
This is causing concern among Kurdish fighters who have been fighting ISIL in northern Iraq.
“We are afraid that after the liberation there will be a force whose mechanism doesn’t go along with the people of Mosul, a force that doesn’t have close ties with the people of Mosul, that is from a different sect. Not from the Sunni sect ” said Sheikh Lukhman Sharawani, a Kurdish militia commander in Iraq.
“We are afraid that they will do to the people of Mosul exactly what they did to the people of Ramadi, and therefore we would like them to keep their distance from Mosul.”
Last month, Human Rights Watch asked Iraqi military commanders to exclude militias from the upcoming battle for Mosul.
“Iraqi commanders shouldn’t risk exposing Mosul civilians to serious harm from militias with a record of recent abuse,” said Joe Stork, deputy Middle East director.
A million people are estimated to be in Mosul and the United Nations says the operation towards the city has already displaced a hundred thousand people.
Aid agencies are struggling to provide help to the nearly three and a half million Iraqis already forced from their homes.
As temperatures rise, life in poorly equipped camps becomes even more difficult.