By Phil Stewart and Yeganeh Torbati

WASHINGTON (Reuters) - The United States said on Tuesday it was deploying a new force of special operations troops to Iraq to conduct raids against Islamic State there and in neighboring Syria, a ratcheting up of Washington's campaign against the group that was quickly rejected by Iraq's government.

Iraqi Prime Minister Haider al-Abadi said the deployment of such a force was not acceptable without Iraq's approval, raising questions over how closely Washington coordinated the plan with Baghdad. Powerful Shi'ite Muslim armed groups pledged to fight any new deployment of U.S. forces to the country.

U.S. Defense Secretary Ash Carter said the deployment of the new "specialized expeditionary targeting force" was being carried out in coordination with Iraq's government and would aid Iraqi government security forces and Kurdish peshmerga forces.

"These special operators will over time be able to conduct raids, free hostages, gather intelligence and capture ISIL leaders," Carter told the U.S. House of Representatives Armed Services Committee, using an acronym for Islamic State.

"This force will also be in a position to conduct unilateral operations into Syria."

While the force is expected to number only about 200, its creation marks the latest stepping up of U.S. military pressure on Islamic State while also exposing American forces to greater risk, something President Barack Obama has done only sparingly.

The force is separate from a previously announced deployment of up to 50 U.S. special operations troops in Syria to coordinate on the ground with U.S.-backed rebels fighting in a civil war raging since 2011.

"The Iraqi government stresses that any military operation or the deployment of any foreign forces - special or not - in any place in Iraq cannot happen without its approval and coordination and full respect of Iraqi sovereignty," Abadi said in a statement.

Carter offered few details on the new group, whose mission promises a more regular operational role for U.S. special forces than seen since the return of American troops to Iraq last year following their withdrawal in 2011 after nearly nine years of war.

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During the congressional testimony in which he disclosed the creation of the force, Carter declined to say how many U.S. troops would be deployed. One U.S. official, speaking on condition of anonymity, said the force may number around 200 troops including support personnel, with only several dozen likely to conduct operations.

'A PRIMARY TARGET'

Jafaar Hussaini, a spokesman for Kata'ib Hezbollah, one of the main Shi'ite militant groups, said that any such U.S. force would become a "primary target for our group."

"We fought them before and we are ready to resume fighting," he said.

Obama in August 2014 authorized the first U.S. air strikes in Iraq since the 2011 U.S. troop withdrawal and has deployed more than 3,000 American military forces to train and advise Iraqi and Kurdish forces to fight Islamic State.

Obama is under pressure to accelerate a U.S.-led coalition's efforts to combat Islamic State, in particular after the Nov. 13 Paris attacks that killed 130 people. He has been reluctant to commit large numbers of U.S. ground troops, instead deploying limited numbers of advisors and elite forces.

His critics, including Republicans in Congress, accuse Obama of moving too slowly against Islamic State, which controls large swathes of territory in Iraq and Syria and claimed responsibility for the Paris attacks.

Republican U.S. Senator John McCain said on Tuesday the new force represented more "incrementalism" in the Obama administration's approach toward Islamic State.

"It also indicates we don't have a strategy," McCain told MSNBC, accusing Obama of simply reacting to the Paris attacks.

Referring to Islamic State attacks outside Syria and Iraq, McCain said: "They are metastasizing, and time is not on our side."

The top U.S. military officer, Marine Corps General Joseph Dunford, said the new force would greatly accelerate the collection of intelligence, which "will make our operations much more effective."

"We're fighting a campaign across Iraq and Syria so we're going to go where the enemy is, and we're going to conduct operations where they most effectively degrade the capabilities of the enemy," Dunford testified.

Carter said all coalition members must do more and singled out Turkey, saying it should become more active in the air war against Islamic State, secure its border and go after the militant group's facilitators who "intrude" into the country.

(Reporting by Phil Stewart and Yeganeh Torbati; Additional reporting by Susan Heavey and Mohammad Zargham in Washington, and Ahmed Rasheed in Baghdad; Editing by Will Dunham)