WASHINGTON — The United States likely will need to keep thousands of troops in Iraq beyond 2011 to keep a lid on sectarian tensions and to bolster Baghdad's fledgling military, experts and former officers say.

American officials privately acknowledge that the US military presence in Iraq will almost certainly be extended, even though a security agreement in force requires all US forces to depart by the end of 2011.

The US military will be needed not only for technical tasks to keep the Iraqi armed forces afloat, but as a reassuring presence for Iraqis fearing a revival of sectarian and ethnic bloodshed, analysts said.

Baghdad's military remains heavily dependent on US logistical support, air power, equipment and expertise, while most Baghdad politicians are anxious to retain American troops as a peacekeeping force in reserve.

"The more pressing requirement is less teaching them how to use weapons and more providing reassurance to threatened internal communities that they won't be exploited by their erstwhile internal rivals," said Stephen Biddle of the Council on Foreign Relations.

"What you're trying to do is make the size of the troop presence proportional to the residual fear that the groups feel towards each other," Biddle said.

Delivering technical help while playing a limited peacekeeping role would require a relatively modest number of troops, perhaps as few as three brigades or roughly 10,000 troops, several former military officers said.

"I think it could get down to even less than 10,000 and still be viable," John Ballard, a professor at National Defense University and a retired army officer, told AFP.

Nearly 50,000 US troops are now in Iraq under an "advise and assist" role, after President Barack Obama on Tuesday declared a formal end to the US combat mission.

The White House, keen to wind down the US role in Iraq, has played down the possibility of a large US force. Vice President Joe Biden's national security advisor, Anthony Blinken, has said only "dozens or maybe hundreds" of troops could remain.

But Iraqi army chief of staff General Babaker Zebari told AFP last month that his country's forces would require US support for another decade, while some analysts in Washington argue for keeping about half of the current force after 2011.

Iraq's "leaders are likely to ask that tens of thousands of American troops stay on for an extended period," Richard Haas, a top diplomat during George W. Bush's presidency, wrote Thursday.

Beyond 2011, the US military would be needed to provide badly-needed logistical support for an army that has been designed mainly as a counter-insurgency force.

The United States would provide some fire power, helicopters, fighter jets to defend a country with virtually no air force, naval defenses for ports and coveted intelligence collected from unmanned robotic planes.

The mission likely would include US special forces assisting Iraqis in manhunts of Al-Qaeda figures, according to James Danly of the Institute for the Study of War, who served in Iraq as an officer.

Apart from operational and tactical support, a US force also would have to be prepared for possible worst case scenarios, Danly and other analysts said.

If relations between the country's Sunnis, Shiites and Kurds threatened to spiral out of control, or if vital oil or other infrastructure came under threat from within or outside Iraq, Baghdad could turn to the US force for help, he said.

In addition to soldiers in uniform, US officials are planning to employ thousands of private contractors to take up security duties formerly performed by troops.

Any talks on the future of the US presence will have to wait for a new government in Iraq, where politicians have failed to agree a power-sharing deal since parliamentary elections in March.

Forging agreement on a post-2011 US mission would present a delicate political challenge for Iraq, as leaders there privately back a continued presence but are reluctant to publicly endorse it.

"It's going to be very hard for any government in Iraq to negotiate anything sizable or enduring," said Ballard. "This puts us in a difficult situation."

The current security accord signed in 2008 was negotiated under a shroud of secrecy, he said, and a follow-on mission also would have to be agreed discreetly, perhaps without a detailed, long-term agreement.

"There's a need, there's a rationale. But it's going to be difficult to put it in any sort of formal way."