When the wall is finished, the minority Sunni community of Azamiyah, on the eastern side of the Tigris River, will be gated, and traffic control points manned by Iraqi soldiers will be the only entries, the military said.
"Shiites are coming in and hitting Sunnis, and Sunnis are retaliating across the street," said Capt. Scott McLearn, of the U.S. 407th Brigade Support Battalion, which began the project April 10 and is working "almost nightly until the wall is complete," the statement said.
It said the concrete wall, including barriers as tall as 12 feet, "is one of the centerpieces of a new strategy by coalition and Iraqi forces to break the cycle of sectarian violence" in Baghdad.
As the wall went up, Secretary of Defense Robert Gates continued his surprise trip to Iraq by hammering home his message that the U.S. was not interested in an open-ended presence in the country.
"Our commitment to Iraq is long-term, but it its not a commitment to having our young men and women patrolling Iraq's streets open-endedly," Gates said at a news conference in Baghdad.
Gates said he encouraged the Iraqis to pass legislation on political reconciliation and the sharing of oil revenues among the Sunnis, Shiites and Kurds. He told them whether they take action on these measures will be taken into consideration when he and the commanders review the military buildup later this summer.
Gates said Prime Minister Nouri al-Maliki assured him that he and the governing council want to "work very hard" to bring about these changes, but also reminded Gates that the council is an independent body.
But as the U.S. troops rushed to build a wall around Sunnis in the middle of Baghdad, little evidence of Iraq's various sects beginning to accept each other was to be found on the streets of the capital city.
U.S. and Iraqi forces have long erected cement barriers around marketplaces and coalition bases and outposts in Baghdad and other Iraqi cities such as Ramadi in an effort to prevent attacks, including suicide car bombs.
American forces also have constructed huge sand barriers around towns such as Tal Afar, an insurgent stronghold near the Syrian border.
There has been little sign, however, of the U.S. military using concrete barriers to divide Baghdad neighborhoods by sect, but at least one similar construction has been reported in the capital.
U.S. Maj. Gen. William B. Caldwell, the top spokesman for coalition forces in Iraq, was quoted as saying Wednesday that he was unaware of any effort to build a wall dividing Shiite and Sunni enclaves in Baghdad and that such a tactic was not a policy of the Baghdad security plan.
"We have no intent to build gated communities in Baghdad," Stars and Stripes, the U.S. Department of Defense-authorized daily newspaper, quoted Caldwell as saying. "Our goal is to unify Baghdad, not subdivide it into separate (enclaves)."
Currently, the U.S. strategy for stabilizing Iraq involves getting Iraqis to reconcile and support the democratically elected Shiite-dominated government in Baghdad, and a security plan in the capital that calls for 28,000 additional American troops and thousands of Iraqi soldiers.
The Azamiyah barrier will allow authorities to screen people entering and leaving the area of northern Baghdad "while keeping death squads and militia groups out," the U.S. military statement said.
Security in the three Shiite communities on the other side of the wall also will be stepped up, and the barrier is expected to make it harder for insurgents to plant roadside bombs in the area targeting coalition forces, the military said.
The construction work by the U.S. military involves flatbed trucks carrying concrete barriers weighing 14,000 pounds. Operating under bright lights, the cranes lift the barriers into place while being protected by U.S. tanks.
As work continued Friday, the day of worship in mostly Muslim Iraq, several Sunnis living in Azamiyah welcomed the effort to improve their security, but said the wall was another sign of the deep hostility between Sunnis and Shiites.
"It is good from one hand to curb violence and have control of terrorists. But it's bad on the other hand to be separated from others. We should live in one area like brothers, not be separated from one another," said Bashar Abdul Latif, a 45-year-old teacher.
"I don't think this wall will solve the city's serious security problems," said Ahmed Abdul-Sattar, 35, a government worker. "It will only increase the separation between our people, which has been made so much worse by the war."
In other developments: