A Shi'ite paramilitary force said late on Tuesday it will support the Iraqi army's offensive on Mosul, Islamic State's last major stronghold in Iraq, raising the risk of sectarian strife in the mainly Sunni region.



The Popular Mobilization Force (PMF), a coalition of mostly Iranian-trained militias, said it will back government forces advancing toward Tal Afar, about 55 kilometers (34 miles) west of Mosul.



Tal Afar's population was a mix of Sunni and Shi'ite ethnic Turkmens until Shi'ites fled the town after Islamic State's ultra-hardline Sunni militants took over the region in 2014, declaring a ''caliphate'' over parts of Iraq and neighboring Syria.



The PMF also said it will be supporting the armed forces advancing into the city of Mosul from the west.



"The PMF will be backing the security forces on the western front (..) along two axis, the first is Tal Afar and the second is to support the forces going into the center of Mosul," a statement on the paramilitary force's website said.



The announcement came despite warnings from human rights groups that PMF involvement could ignite sectarian violence.



Shi'ites make up a majority in Iraq but Sunnis are predominant in the north and the west.



The PMF officially reports to the Shi'ite-led government of Prime Minister Haider al-Abadi, who announced on Monday the start of the offensive on Mosul, Iraq's second-largest city.



Abadi had sought to allay fears of sectarian bloodshed, saying that the army and the police will be the only forces allowed to enter the city.



The PMF was formed in 2014 to help push back Islamic State's sweeping advance through the northern and western provinces.



Amnesty International on Tuesday published a report saying Shi'ite militias had committed ``serious human rights violations, including war crimes'' against civilians fleeing Islamic State-held territory.



The U.N in July said it had a list of more than 640 Sunni Muslim men and boys reportedly abducted by a Shi'ite militia in Falluja, a former militant stronghold west of Baghdad, and about 50 others who were summarily executed or tortured to death.



The government and the PMF say a limited number of violations had occurred and were investigated, but they deny abuses were widespread and systematic.