Powerful Iraqi Shi'ite Muslim armed groups on Tuesday pledged to fight any US forces deployed in the country after the United States said it was sending an elite special unit to help combat Islamic State.Defense Secretary Ash Carter offered few details on the new "expeditionary" group, but said it would be larger than the roughly 50 US special operations troops being sent to Syria to fight the ultra-hardline Sunni militants there.A US defense official, speaking on condition of anonymity, said the new force will be based in Iraq."We will chase and fight any American force deployed in Iraq," said Jafaar Hussaini, a spokesman for one of the Shi'ite armed groups, Kata'ib Hezbollah. "Any such American force will become a primary target for our group. We fought them before and we are ready to resume fighting."Spokesmen for the Iranian-backed Badr Organisation and Asaib Ahl al-Haq made similar statements to Reuters, expressing their distrust of American forces since the 2003 US-led invasion that toppled Saddam Hussein and the subsequent occupation.