Iraqi Shi'ite militias pledge to fight U.S. forces if deployed

Defense Secretary Ash Carter offered few details on the new "expeditionary" group, but said it would be larger than the roughly 50 U.S. special operations troops being sent to Syria to fight the ultra-hardline Sunni militants there.

A U.S. 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 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 after the 2003 U.S.-led invasion that toppled Saddam Hussein and the subsequent occupation.

The militias, grouped with volunteer fighters under a government-run umbrella, are seen as a bulwark in Iraq's battle against Islamic State, the biggest security threat to the oil-exporting country since Saddam's fall.

Russia's larger military role in neighboring Syria, and its participation in a security coordination cell in Baghdad that includes Iran and Syria, may be deepening U.S. fears that it is losing more strategic ground to rivals in one of the world's most critical regions.

(Reporting by Ahmed Rasheed; Writing by Stephen Kalin; Editing by Mark Trevelyan)