Some of the groups, which have members who fought alongside al-Qaeda in the past, have been providing useful intelligence to U.S. forces about their former allies, according to the U.S. military.
"They know where they live and who they are," said Lt. Col. Rick Welch, a staff officer who works with tribes in the capital area. "They know how they operate." Some tribes are also taking up arms against al-Qaeda allies. About 100 tribes live in greater Baghdad. Many of these clans are groups of relatives who share the same name and have thousands of members.
U.S. commanders have reached similar deals in Sunni-dominated Anbar province in western Iraq. Attacks there have dropped by 60% in the last year, according to the U.S. military. Tribes in Diyala province north of Baghdad are also negotiating with U.S. forces, which have launched a major offensive in the region.
Most of the Baghdad tribes cooperating with U.S. forces are Sunni, Welch said, but he didn't have a specific breakdown.
Iraqi Prime Minister Nouri al-Maliki said this week that his government objected to the arming of various tribes "because this will create new militias."
U.S. commanders are urging al-Maliki to bring the tribes into the legitimate security forces in order to avoid creating militias outside government control. "The goal is to tap into the movement, but not create a threat," Welch said. "If it continues and the government doesn't thwart it, it will be a huge event."
Iraq's government has started to recruit some Baghdad tribe members into the police, Welch said.
Tribal leaders in Baghdad are less influential than in Anbar.
"In Anbar, tribal engagement appears to be the answer," said Lt. Col. Douglas Ollivant, chief of plans for the American division in Baghdad. "In Baghdad it's not going to be the answer. It's going to be part of the answer."
The United States will keep working with al-Maliki's government, Defense Secretary Robert Gates said during a weekend visit to Baghdad, but it can't ignore the result from such "ground up" negotiations with tribes.