By Andrew North
BBC News, Baghdad
The US authorities in Iraq are trying to step up pressure on Iraqi politicians to tackle the power of armed militia groups, increasingly seen as one of the main sources of instability and violence.
Militia violence has affected thousands of Iraqis
A senior US military official has called for the Iraqi government to adopt a clearer policy on militias, which the US embassy now dubs "armed gangs", underlining recent criticism of such groups by the US ambassador.
Yet many also see confusion in US military and diplomatic policy on the issue, with concerns about stability apparently preventing stiffer action.
As with so many other countries afflicted by violence, dealing with these groups may prove to be one of the toughest problems of all.
Members of one Shia militia have been widely implicated in the sectarian violence which has claimed hundreds of lives over the past few weeks, particularly in Baghdad.
But the killings continue with no sign that the authorities are either able or willing to take any action.
One reason, many Iraqis complain, is that these groups have official ties, including to some of the main parties involved in ongoing negotiations to form a government. They have also infiltrated many police units.
In what has become an almost daily ritual, another 15 corpses - believed to be the latest victims of this communal strife - were found by US and Iraqi troops around Baghdad on Saturday.
Their hands were tied and they had suffered gunshot wounds.
Militias "are a problem and they are going to have to be dealt with" said the senior military official, who spoke on condition his name or rank not be identified.
He refused to name the groups he is concerned about, but acknowledged there is confusion about the official stance on militias.
"It's something that there needs to be a clear-cut policy on," he said.
It is partly because of the existence of armed groups that people in Baghdad "do not feel they have adequate security".
Last weekend US ambassador Zalmay Khalilzad lashed out at militia groups.
"More Iraqis are dying from the militia violence than from the terrorists," he told reporters. "The militias need to be under control."
Mr Khalilzad has also threatened in the past to cut off US aid to ministries regarded as following a sectarian agenda, a comment seen as being directed at the Shia Interior Minister Bayan Jabr.
Yet, like the US military official, Mr Khalilzad would not name the militias he was referring to.
But few doubt one of his chief targets was the Mehdi army militia, loyal to the radical cleric Moqtada Sadr.
He has a stronghold of support in Sadr City, a Shia-dominated area of Baghdad.
It is believed that a Mehdi army group was the target of a US-backed Iraqi special forces raid last Sunday in Baghdad, in which at least 16 people were killed and a hostage rescued.
The operation sparked controversy after Shia politicians accused the troops involved of killing innocent civilians in a mosque.
Yet although the US military official reiterated statements by other commanders that it was a legitimate operation on a "terrorist cell", he cited operational security and would not comment on the identity or religious affiliation of those killed, or of the 18 people detained.
There is also concern in southern Iraq over the extensive influence of the Badr brigades, a militia force linked to the largest party in the governing Shia Alliance.
Under a law introduced by the Coalition Provisional Authority, the US-led body which ran the initial occupation of Iraq, all militia groups are supposed to have been disbanded by now.
Order 91, as it was known, called for 90% of the 100,000 militia fighters estimated to be operating at the time to have been demobilised by the time of the January 2005 elections.
In fact these groups have got stronger. And what happens has become an issue in the drawn-out negotiations on forming a government.
Many Iraqis fear militia elements are operating with official sanction.
Men dressed in official uniforms have been accused of atrocities
In Baghdad, there have been a series of unexplained abductions and raids in recent weeks by people reportedly wearing uniforms of government forces.
So bold have these been that few believe they could have been carried out by insurgents.
In one instance in early March, up to 50 members of a private security company were taken away in broad daylight by people in police uniforms, according to eyewitnesses.
The US military official would not comment on whether he thought Iraqi government officials were involved in some of these incidents.
But he said US troops had detained a unit of Iraqi highway police in one case where they believed they were involved in militia activity.
Political requirements have made the militia issue an ever trickier one for both the US and Iraqi governments to deal with - and one that leaves the Americans open to accusations of double standards.
If they have to demobilise their militias, Shia groups say the same should apply to the Kurdish-controlled Peshmerga forces in northern Iraq.
But because they are long-time US allies who backed the 2003 invasion, the US has taken a more lenient approach towards the Kurds.
As in so many other countries affected by conflict, such as Afghanistan or the Balkans, dealing with the unofficial armed groups that spring up is often one of the hardest challenges.
In the current climate of sectarian tension in Iraq, it is even more so.
There is irony here - it is widely reported that the Iraqi special forces unit which led last weekend's raid were drawn from the ranks of the Peshmerga.