The United States said on Tuesday it was deploying a new force of special operations troops to Iraq to conduct raidsagainst Islamic State there and in neighbouring Syria, in a ratcheting up of Washington's campaign against the group.

US 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 US House of Representatives Armed Services Committee.

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

Iraqi Prime Minister Haider al-Abadi's office issued a statement saying it welcomed foreign assistance but Iraq's government would need to approve any deployment of special operations forces anywhere in Iraq – a point Carter also acknowledged.

Abadi reiterated that foreign ground combat troops were not needed in Iraq, although it was unclear whether Baghdad viewed these special operations forces in that role.

Powerful Iraqi Shi'ite Muslim armed groups pledged to fight any such deployment of US forces to the country.

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

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

US officials, speaking on condition of anonymity, said the plan to deploy the special operations forces unit to Iraq was discussed and agreed with the government of Iraq before Carter's announcement.

"As we further develop plans for these limited forces, we will continue to work closely with our Iraqi partners on where they will be deployed, what kind of missions they will undertake, and how they will support Iraqi efforts," one US official said.

Expanding US operations

While the force is expected to number only about 200, its creation marks the latest stepping up of US 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 US special operations troops in Syria to coordinate on the ground with rebels fighting in a civil war raging since 2011.

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

Fred Hof, formerly a top State Department adviser on Syria, said the planned deployment of special forces to Iraq to conduct raids and gather intelligence represented a shift by Obama.

Hof, who is now at the Atlantic Council think-tank, said the US special forces were filling a gap in capable ground forces to fight Islamic State, which won control of Iraq's Anbar provincial capital of Ramadi in May and also controls the northern of Mosul.

Obama "is edging as closely as he can to try to fill this ... gap without violating in a technical sense his own 'boots on the ground' dictum," Hof said, referring to Obama's frequent pledges not to return to US troops to a large-scale ground war.

That pledge, he said, does not appear to apply to US special operations forces. Obama in August 2014 authorized the first US air strikes in Iraq since the 2011 US troop withdrawal and has deployed more than 3,000 American military forces to train and advise Iraqi and Kurdish forces to fight Islamic State.

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

Carter offered few details on the new group, whose mission promises a more regular operational role for US special forces than seen since the return of American troops to Iraq last year.

During the congressional testimony in which he disclosed the creation of the force, Carter declined to say how many US troops would be deployed.

One US 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.

The top US 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.