Gary Samore, an analyst at London's International Institute of Strategic Studies (IISS), said on Wednesday that "it's a sad fact of life that the coalition is not capable of disarming the militias".
"In fact in order to manage the situation," he said, "the coalition has had to accept the creation of new private armies as in the case of Falluja where for all intents and purposes security has been put into the hands of a Sunni, former Baathist force."
A year ago, Iraq's US occupation administrator Paul Bremer dissolved Saddam Hussein's armed forces to root out Baathist influence, vowing that ethnic and sectarian militias would also be disbanded as national security forces were rebuilt.
But today, Kurdish and Shia militias remain largely intact, even if some have removed or changed their uniforms.
A new militia led by Shia leader Muqtadar al-Sadr is fighting US-led forces in the south. And in Falluja, a local force, led by former Iraqi army officers and incorporating resistance fighters, has taken over in a deal with the US military.
"This proliferation of militias in Iraq is the major problem facing the new government," the IISS said in its annual survey released on Tuesday.
"If the security vacuum cannot be dealt with, then ordinary Iraqis will increasingly be required to look to the militias for some sense of order. In return, these militias will demand political loyalty from their new constituents"
"If the security vacuum cannot be dealt with, then ordinary Iraqis will increasingly be required to look to the militias for some sense of order. In return, these militias will demand political loyalty from their new constituents."
This does not imply a Lebanon-style sectarian civil war is imminent, the survey argues, but if their power is unchecked, militias could undermine central government authority.
The US military's crackdown on al-Sadr's Mahdi Army appears to have been a belated - and not yet successful - attempt to eliminate a militia trying to disrupt Washington's aims to hand over to a pro-American government on 30 June.
US-led forces, now more than 150,000 strong, will stay in Iraq after the handover to try to prevent violence from wrecking plans to hold national assembly elections by January.
The United States and Britain have drafted a Security Council resolution that calls on UN members to contribute to the multinational force, but few will rush to do so.
"It's very unlikely that large numbers of additional forces will be sent to Iraq," said Christopher Langton, an IISS military analyst, estimating that up to 500,000 troops were needed to deal with Iraq's numerous, well-armed resistance.
US and British forces are stretched thin and faced strong domestic constraints on expanding their role. Other nations are not keen to send troops into "this desperate situation".
Many Iraqis have no respect for
the US-appointed authorities
It would be hard for UN envoy Lakhdar Brahimi to pick an interim government of Iraqis with national appeal, rather than those representing ethnic and sectarian groups, Langton said.
Absence of leadership
"The long-term concern is that the combination of increasing strength of the militias and the absence of national political leadership in Iraq may create the conditions for civil strife."
Waiting uneasily in the wings are Kurdish and Shia parties which have cooperated with the US-led occupation but do not want to disarm while insecurity and uncertainty prevail.
Thousands of Kurdish peshmerga militiamen in the north have joined the police or other units of the new security forces, but their main loyalties lie with the two main Kurdish parties.
The armed wings of two Shia groups, the Badr Brigade and the Daawa party militia, have largely stayed out of sight or changed their names, but they have hung onto their guns.
These Kurdish and Shia groups have long argued their trained fighters and intelligence men could deal effectively with Baathist rebels and foreign fighters if given the chance.
But wary of entrenching militias in Iraq's security forces, the US-led administration has resisted. In a new twist, it announced last month that former Iraqi officers were welcome to join the new army, provided they did not have "blood on their hands".
While many Iraqis distinguish the old regular army from Saddam's hated Republican Guards and security agencies, some may have misgivings about swiftly reconstituting it now.
President George Bush himself has acknowledged failures.
"The long-term concern is that the combination of increasing strength of the militias and the absence of national political leadership in Iraq may create the conditions for civil strife"
IISS military analyst
"In some cases, the early performance of Iraqi forces fell short. Some refused orders to engage the enemy," he said this week, adding that units would get more training in future.
Bush spoke of plans to train 260,000 Iraqi soldiers, police, and other security personnel, saying the goal was an army of 35,000 in 27 battalions. Five battalions are already in the field and another eight will join them by 1 July.
Defence Minister Ali Allawi said on Tuesday that Iraqi forces, stiffened by recruits from the Saddam-era army, could gradually take on a bigger role and replace US-led troops within a year.
However, the IISS said rebuilding Iraqi forces was a long-term venture, which could be compromised by relying on militias or integrating them too fast.
"There appears to be little chance in the immediate future that the security vacuum... can be filled by either coalition troops or by the nascent military and police forces hastily stood up since liberation," the survey said.