WASHINGTON: The United States has yet to agree with Baghdad on crucial details governing the role of a new American special forces unit aimed at hunting Islamic State militants in Iraq, US officials said, underlining the difficulties Washington faces dealing with Iraq's weakened leader.

Defense Secretary Ash Carter announced on Tuesday the planned deployment of the small force, whose raids against Islamic State targets would be the first sustained military operations by US forces in Iraq since American combat troops left in 2011.

US officials said it had been discussed and coordinated with Iraqi Prime Minister Haider al-Abadi.

But Iraq's ruling alliance and powerful Shi'ite militias have warned against the plan, raising doubts over whether Abadi has the political clout to secure a final agreement.

In comments that may have been intended mostly for public consumption at home, Abadi said hours after Carter's announcement that any such deployment would require his government's consent. On Thursday, he said that any deployment of foreign ground troops would be considered an "act of aggression."

US officials stressed that there will be no unilateral American military operations in Iraq, unlike in neighbouring Syria. But precisely how much say Abadi will have over the unit's activities, and how much freedom of action the Americans will have, is still undecided. Having Abadi sign off before each raid would be cumbersome, US officials believe, and crimp the new unit's effectiveness.

The Obama administration plans to send a team to Baghdad in coming weeks to sort out the details with Iraq's government, officials said.

"With Abadi, a core fundamental principle of ours in this whole thing is that everything we do in Iraq is with full consent and coordination with the Iraqi government," a senior administration official said. "So we will not be doing anything in Iraq unilaterally."

It is unclear if the unresolved questions will prompt a delay in the dispatch of about 100 elite US Special Operations Forces to Iraq, which Carter said would launch raids in both Syria and Iraq to secure hostages, gather intelligence and capture Islamic State leaders.

US DILEMMA OVER ABADI

The strong resistance to the plan in Iraq highlights a dilemma for US President Barack Obama.

He wants to do more to fight Islamic State, responsible for the recent deadly attacks in Paris, Egypt and elsewhere, and which controls swathes of Syria and Iraq. But he also does not want to undermine US ally Abadi, whose power is severely circumscribed by the Shi'ite militias.

Abadi has been under mounting US pressure to rein in the Iranian-backed armed groups, angering the forces who enjoy support from many of Iraq's majority Shi'ites and which have also been a bulwark against Islamic State.

Under one option being considered for the force, Abadi and his government would give their assent for the US special operators to conduct raids in a given area against a pre-agreed list of targets.

That would allow US forces to be more nimble-footed in acting on time-sensitive intelligence. Abadi would likely be notified before, or as, each raid is launched - but not sign off on a mission-by-mission basis.

"There are ways to make these things work," said the senior official, speaking on condition of anonymity because of the issue's sensitivity.

Another key question is whether Baghdad will be brought into the picture on raids the United States conducts exclusively with Kurdish Peshmerga forces, given that the Kurds often insist on acting independently from the Iraqi military.

The likelihood, the US officials said, is that Abadi would at least be briefed in advance on high-profile operations with Kurdish fighters -- as occurred with an October rescue mission that freed dozens of hostages from an Islamic State jail -- but not so for more routine raids.

In Baghdad, where memories of the US occupation remain fresh, Shi'ite lawmakers have threatened to question Abadi in parliament over the planned American deployment, and even to seek a no-confidence vote in his leadership.

Calls by two senior US senators, Republicans John McCain and Lindsey Graham, while visiting Baghdad on Nov. 29 to triple US force levels in Iraq to 10,000 -- with an equal number in Syria -- also heightened Iraqis' suspicions.

"If they stopped giving these statements and they do it with the government, they could send not just 100 - maybe 500 without anyone rejecting," said Sami Askari, a senior lawmaker from Abadi's State of Law coalition.

"But when they come in public and say we will send 10,000, we need to send ground troops, everyone will say what’s going on?" Askari said.

Washington recognises that Carter's announcement could add to Abadi’s political difficulties, another US official said.

The Obama administration hopes that its expressed willingness to consult with Abadi on the deployment could help keep Shi’ite hardliners at bay, the official said.

Secretary of State John Kerry said on Wednesday that "We will continue to work very, very closely with our Iraqi partners on exactly who would be deployed, where they would be deployed, what kinds of missions people would undertake."