By Stephen Kalin and Michael Georgy
SOUTH OF MOSUL, Iraq (Reuters) - Iraqi troops and security forces edged closer to Mosul on two southern fronts on Sunday but a leader of the Shi'ite militias newly participating in the offensive warned that the battle for Islamic State's Iraq stronghold would be long and grueling.
A military statement said the army's Ninth Armoured Division raised the Iraqi flag in the village of Ali Rash, about 7 km (4 miles) southeast of Mosul, after recapturing it from the ultra-hardline Sunni Muslim militants.
Further south, an Interior Ministry officer said security forces were advancing from the town of al-Shura, recaptured from Islamic State (IS) on Saturday, along the Tigris river valley towards Mosul 30 km (20 miles) to the north.
The army and security forces, along with Kurdish peshmerga fighters, have been backed by U.S.-led air and ground support in their two-week-old campaign to crush Islamic State in the largest city of its self-declared caliphate in Iraq and Syria.
Their battle for Mosul, still home to 1.5 million residents, could be one of the toughest in a decade of turmoil since the 2003 overthrow of Saddam Hussein, a Sunni Muslim, brought Iraq's majority Shi'ites to power.
On Saturday thousands of Iranian-backed Iraqi Shi'ite militia fighters, known as the Hashid Shaabi (Popular Mobilisation) forces, joined the Mosul offensive, launching a campaign to take territory to the west of the city.
Their target is to seize the town of Tal Afar, 55 km (35 miles) west of Mosul, from Islamic State.
That would cut off any chance of the jihadists retreating into - or being reinforced from - their positions in neighboring Syria, Ahmed al-Asadi, a spokesman of the Popular Mobilisation, told a news conference. IS fighters have been "flowing into Mosul" from Syria, he added.
Iraqi troops and Kurdish peshmerga fighters are already driving Islamic State fighters back on the southern, eastern and northeastern approaches to Mosul.
"There is cooperation between ... the army, federal police, Hashid and counter-terrorism (forces) and also the (local Sunni) tribes," said Hadi al-Amiri, head of the Badr Organisation, the most powerful group within the Popular Mobilisation forces.
Speaking in the village of Ain Nasir in the semi-arid land west of the Tigris, Amiri said the fight against Islamic State insurgents holding Mosul, who have already launched waves of suicide car bombs, roadside bombs and sniper attacks to slow down the advancing forces, could be long and bloody.
"The battle of Mosul will not be a picnic. It needs time, it needs precision, it needs a deep breath," he said, wearing military fatigues and with his face wrapped in a white checked headscarf against the wind and sand.
"We are prepared for the battle of Mosul even if it lasts for months".
ISLAMIC STATE "REINFORCING" TAL AFAR
The deployment of Shi'ite forces in northern Iraq, an ethnically mixed region where Sunni Muslims form a majority, could inflame sectarian tensions and has led to warnings from neighboring Turkey.
President Tayyip Erdogan said the town of Tal Afar, which the Shi'ite forces say they will recapture, is Turkmen - inhabited by people with strong cultural and historical links to Turkey - and said Ankara would act if the Popular Mobilisation forces "unleash terror" there.
Anticipating the offensive on Tal Afar, and highlighting its strategic importance, Islamic State has also been reinforcing the town in the last 48 hours, an Iraqi security official said.
He said two waves of reinforcements were sent including insurgents who had fought in neighboring Syria and had experience in using anti-tank missiles.
"Intelligence reports show that the Daesh (Islamic State) groups have entered TOWmissiles systems into Tal Afar. It's obvious they are making preparations for a long protracted battle," the official from the provincial military operation command center told Reuters.
His comments could not be independently verified but a resident of Mosul, speaking to Reuters by phone, said relatives in Tal Afar reported seeing increasing numbers of Islamic State fighters in the town, some of them patrolling on motorbikes.
Since launching their advance towards Tal Afar on Saturday, the Popular Mobilisation forces have taken over several villages in an area about 60 km (40 miles) southeast of their final target.
The Kurdish Peshmerga said in a statement on Sunday they had cleared at least 28 villages north and east of Mosul since the start of the offensive.
Interior Ministry rapid response forces, who took control on Saturday of the town of al-Shura, about 30 km (20 miles) south of Mosul, advanced on Sunday a few km northeast and took three villages from Islamic State, an officer told Reuters.
The Popular Mobilisation force, formed in 2014 to help push back Islamic State's sweeping advance, officially report to Prime Minister Haidar al-Abadi's Shi'ite-led government, but has very close links to Iran.
Human rights groups have warned of possible sectarian violence if the Shi'ite paramilitaries seize areas where Sunni Muslims form a majority, which is the case in much of northern and western Iraq.
(Additional reporting by Michael Georgy in Erbil, Iraq; writing by Dominic Evans; editing by Mark Heinrich)