By Maher Chmaytelli and Babak Dehghanpisheh
BAGHDAD/ERBIL, Iraq (Reuters) - A Shi'ite paramilitary force said it would 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 late on Tuesday it would back Iraqi government forces advancing toward Tal Afar, about 55 kilometers (34 miles) west of Mosul.
Taking Tal Afar would effectively cut off the escape route for militants wanting to head into neighboring Syria and would please the Iran-backed army of Syrian President Bashar al-Assad. It has accused the U.S.-led anti-Islamic State coalition of planning to allow the jihadists such a safe passage.
But it could also hamper the escape of civilians from the area of Mosul, which is Iraq's second biggest city and where the militants are reportedly trying to use residents as human shields.
Iraqi Prime Minister Haider al-Abadi said on Tuesday that blocking the road to Syria was the responsibility of the coalition that is providing air and ground support to Iraqi and Kurdish troops engaged in the battle.
Tal Afar's pre-war population of about 150,0000 to 200,000 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 swathes of Iraq and Syria.
"The Iranians and the (PMF) plan to take Tal Afar because of the Shi’ite significance and use that as a way to angle in to Mosul," said a senior Iraqi official who asked not to be named. "But they also want to use it as a way to angle into the Syria fight."
Iraqi government forces are mostly fighting on the southern front and were trying on Wednesday to clear the region of Shora, 40 km (25 miles) south of the city, a military statement said. Kurdish Peshmerga fighters were deployed on the eastern and northern frontlines, 20 to 25 km from the city.
"The PMF will be backing the security forces on the western front (..) along two axes. The first is Tal Afar and the second is to support the forces going into the center of Mosul," a statement on the PMF 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 of the country.
The PMF officially reports to the Shi'ite-led government of Abadi, who announced on Monday the start of the Mosul offensive. It was formed in 2014 to help push back Islamic State's sweeping advance through the northern and western provinces.
Abadi has sought to allay fears of sectarian bloodshed, saying that the army and the police will be the only forces allowed to enter the city.
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 United Nations said in July 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 did occur and were investigated, but they deny the abuses were widespread and systematic.
Iraqi officials and residents of Mosul say Islamic State is preventing people from leaving the city but civilians are fleeing from outlying districts and villages.
The United Nations has warned that some 100,000 people may arrive in Syria from the Mosul area. Save the Children said on Wednesday about 5,000 people, mostly women and children, had arrived at the Al Hol camp in Syria in the last 10 days.
"At least a thousand more are now massing at the border waiting to cross," the organization said in a statement.
(Reporting by Maher Chmaytelli; Editing by Gareth Jones)