Although the American engagement with the Sunni groups has brought some early successes against Al Qaeda, particularly in Anbar, many of the problems that hampered earlier American efforts to reach out to insurgents remain unchanged. American commanders say the Sunni groups they are negotiating with show few signs of wanting to work with the Shiite-led government of Prime Minister Nuri Kamal al-Maliki. For their part, Shiite leaders are deeply suspicious of any American move to co-opt Sunni groups that are wedded to a return to Sunni political dominance.

With the agreement to arm some Sunni groups, the Americans also appear to have made a tacit recognition that earlier demands for the disarming of Shiite militia groups are politically unachievable for now given the refusal of powerful Shiite political parties to shed their armed wings. In effect, the Americans seem to have concluded that as long as the Shiites maintain their militias, Shiite leaders are in a poor position to protest the arming of Sunni groups whose activities will be under close American scrutiny.

But officials of Mr. Maliki’s government have placed strict limits on the Sunni groups they are willing to countenance as allies in the fight against Al Qaeda. One leading Shiite politician, Sheik Khalik al-Atiyah, the deputy Parliament speaker, said in a recent interview that he would rule out any discussion of an amnesty for Sunni Arab insurgents, even those who commit to fighting Al Qaeda. Similarly, many American commanders oppose rewarding Sunni Arab groups who have been responsible, even tangentially, for any of the more than 29,000 American casualties in the war, including more than 3,500 deaths. Equally daunting for American commanders is the risk that Sunni groups receiving American backing could effectively double-cross the Americans, taking weapons and turning them against American and Iraq’s Shiite-dominated government forces.

Americans officers acknowledge that providing weapons to breakaway rebel groups is not new in counterinsurgency warfare, and that in places where it has been tried before, including the French colonial war in Algeria, the British-led fight against insurgents in Malaya in the early 1950s, and in Vietnam, the effort often backfired, with weapons given to the rebels being turned against the forces providing them. Maj. Gen. Rick Lynch, commander of the Third Infantry Division and leader of an American task force fighting in a wide area between the Tigris and Euphrates rivers immediately south of Baghdad, said at a briefing for reporters on Sunday that no American support would be given to any Sunni group that had attacked Americans. If the Americans negotiating with Sunni groups in his area had “specific information” that the group or any of its members had killed Americans, he said, “The negotiation is going to go like this: ‘You’re under arrest, and you’re going with me.’ I’m not going to go out and negotiate with folks who have American blood on their hands.”

One of the conditions set by the American commanders who met in Baghdad was that any group receiving weapons must submit its fighters for biometric tests that would include taking fingerprints and retinal scans. The American conditions, senior officers said, also include registering the serial numbers of all weapons, steps the Americans believe will help in tracing fighters who use the weapons in attacks against American or Iraqi troops. The fighters who have received American backing in the Amiriya district of Baghdad were required to undergo the tests, the officers said.

The requirement that no support be given to insurgent groups that have attacked Americans appeared to have been set aside or loosely enforced in negotiations with the Sunni groups elsewhere, including Amiriya, where American units that have supported Sunni groups fighting to oust Al Qaeda have told reporters they believe that the Sunni groups include insurgents who had fought the Americans. The Americans have bolstered Sunni groups in Amiriya by empowering them to detain suspected Qaeda fighters and approving ammunition supplies to Sunni fighters from Iraqi Army units.

In Anbar, there have been negotiations with factions from the 1920 Revolution Brigades, a Sunni insurgent group with strong Baathist links that has a history of attacking Americans. In Diyala, insurgents who have joined the Iraqi Army have told reporters that they switched sides after working for the 1920 group. And in an agreement announced by the American command on Sunday, 130 tribal sheiks in Salahuddin met in the provincial capital, Tikrit, to form police units that would “defend” against Al Qaeda.