WASHINGTON: The United States (US) military will deploy a new special operations force to Iraq to step up the fight against the militant Islamic State (IS) group unleashing violence in Iraq and Syria and determined to hold territory they have seized across the Middle East, Defence Secretary Ash Carter told Congress on Tuesday.

Carter, who testified alongside Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, faced sceptical lawmakers who argued that the US needs to be more forceful in countering the threat from IS, credited with attacks in Paris and Beirut and the downing of a Russian airliner.

The defence secretary told the House Armed Services Committee that over time, the special operations force will be able to conduct raids, free hostages, gather intelligence and capture IS leaders.

Carter said that will improve intelligence and generate more targets for attacks.

There currently are about 3,500 US troops in Iraq, and President Barack Obama had previously announced he was sending fewer than 50 special operations forces to Syria.

There has been a growing call from some Republicans for more US ground forces and a divide among war-weary Americans about the prospect of greater military involvement.

Carter said the number in the new expeditionary force will be “larger” than 50. He said it will be a "standing" force, meaning it will be stationed in Iraq.

He said it would focus on helping Iraq defend its borders and build the Iraqi security forces, but also would be in position to conduct unilateral operations into Syria.

"This is an important capability because it takes advantage of what we're good at," Carter said.

"We're good at intelligence, we're good at mobility, we're good at surprise. We have the long reach that no one else has. And it puts everybody on notice in Syria. You don't know at night who's going to be coming in the window. And that's the sensation that we want all of IS leadership and followers to have."

According to a US official, the force could total up to a couple hundred troops, including the assault teams, aviation units and other support units. It would likely be based in Irbil.

Obama has set the maximum number of troops at 3,550, but it was not clear whether the president will increase that number to accommodate the force, or whether the teams would have to be built within the current limit.

The official spoke on condition of anonymity because the individual was not authorised to publicly discuss military planning.

Carter said the force might be American-only, but more likely would be a mixed force with perhaps Kurdish troops or others who are fighting the militants. He said the new force would conduct operations similar to two conducted earlier this year.

In October, US special operations troops and Iraqi forces raided a compound in northern Iraq freeing about 70 Iraqi prisoners who were facing execution.

One US service member was killed in the raid, the first American combat death in Iraq since the US began its campaign against IS in August 2014.

In May, a Delta Force raid in Syria killed IS financier Abu Sayyaf, yielded intelligence about the group's structure and finances, and his wife, held in Iraq, has been cooperating with interrogators.