Iraqi Shi’ite militias backed by Iran said on Friday they would soon join the fight against Islamic State on a new front west of Mosul, a move which could block any retreat by the jihadists into Syria but might alarm Turkey and the United States.

The Shi’ite militias, with thousands of battle-hardened fighters trained by Iran, would bring important extra firepower to what is expected to be the biggest battle in Iraq since the U.S.-led invasion in 2003.

But their arrival on the battlefield in one of the most diverse parts of Iraq also creates worry for Western countries backing the Iraqi government offensive, who fear that the Shi’ite fighters could alienate residents in mainly Sunni areas. […]