The 172nd Stryker Brigade, which had been due to leave Iraq after a year's assignment, will be sent from the north to the capital, Gen. George W. Casey Jr., the top U.S. commander in Iraq, said.
"This will place our most experienced unit with our most mobile and agile systems in support of our main effort," Casey said. "With the rest of the elements of the plan, this gives us a potentially decisive capability to affect security in Baghdad."
The statement followed President George W. Bush's decision this week to bolster American forces in Baghdad to try to stem the tide of Sunni-Shiite violence, now seen as a greater threat to Iraq than the Sunni-led insurgency.
"Bagdad is a problem for us, there's no doubt about that. We are fighting Iraqis who are killing Iraqis," said Lt. Gen. Peter Chiarelli, commander, multi-national corps, told CBS News correspondent Mark Strassman.
The U.S. plan calls for moving up to 5,000 additional American troops with armored vehicles and tanks into the capital. Some critics believe the move will undermine confidence among Iraqi forces and expose more U.S. soldiers to attacks by Sunni insurgents and Shiite militias.
The announcement comes a day after the head of the biggest Shiite party called for a greater security role for Iraqis in the country in place of Americans.
Abdul-Aziz al-Hakim, the former commander of the feared Badr Brigade militia, has long complained the Americans have interfered with Iraqi forces' efforts to crack down on Sunni insurgents and al Qaeda in Iraq terrorists.
However, he also endorsed the government's pledge to disband militias, including those affiliated with Shiite politicians.
Members of al-Hakim's Supreme Council for the Islamic Revolution in Iraq have been suspicious of U.S. and Iraqi government peace overtures to Sunni insurgents and have privately complained that top Sunni politicians have intervened to free detainees in Baghdad.
In other developments:
Also on Saturday, coalition forces in Diwaniyah, 80 miles south of Baghdad, made another move against the Mahdi Army, attempting to arrest a senior leader in radical anti-American cleric Muqtada al-Sadr's militia, police said.
In the last month, British troops have arrested the Mahdi commander in the southern city of Basra. And American soldiers killed 15 militiamen in a gunfight 40 miles south of the capital last weekend.
U.S. and Iraqi forces have staged at least two major raids this month in Sadr City, the Mahdi Army's Baghdad stronghold.
The U.S. military announced Saturday that a tip from a resident in Latifiyah, about 20 miles south of Baghdad, led them to detain 25 men believed as suspects in a marketplace attack in Mahmoudiya last week that killed 50 people. The U.S. command also said three U.S. Marines died in action in western Iraq.
The Marines died Thursday in Anbar, the western province that is a focal point of the Sunni-dominated insurgency. A U.S. statement said they were attached to the Army's 1st Armored Division, which operates in Ramadi, but gave no further details.
Their deaths brought the number of U.S. service members who have died since the beginning of the Iraq war in March 2003 to at least 2,573, according to an Associated Press count. The figure includes seven military civilians.
Sectarian violence has escalated in Iraq in recent months, with Sunni radicals, including members of al Qaeda, and Shiite militias staging tit-for-tat killings. Thousands from both sects have fled the country, according to Iraqi officials.
At least 18 were killed Saturday in Iraq, including a Sunni cleric from a tribe opposed to al Qaeda in Iraq who was slain while shot and killed while driving in Samarra, about 60 miles north of Baghdad, police said.
A parked car packed with explosive exploded in a residential district of Kirkuk, killing four people and injuring another 13, police said. It was the sixth car bombing this month in Kirkuk, where tensions are rising among Arabs, Kurds and Turkomen for control of the area's vast oil wealth.
The western regional commander of the Iraqi Border Protection Force, Brig. Gen. Jawad Hadi al-Selawi, was killed in Karbala, 50 miles south of Baghdad, police said.
Iraq's national soccer coach resigned after receiving a death threat, sporting officials said. The country's wresting coach was killed July 13. Two days later, more than 30 sporting officials, including the chairman of Iraq's Olympic Committee, were seized during a meeting in Baghdad.
At least 10 of them have been freed, but dozens are still missing, including National Olympic Committee chairman Ahmed al-Hijiya.
Another top Shiite politician, Hadi al-Amiri, on Friday said there were rumors of a plot to overthrow the government of Prime Minister Nouri al-Maliki, a Shiite, and replace it with a secular "government of national salvation." He did not elaborate.
"We don't call it a national salvation government, we call it a military coup," said al-Amiri, leader of the Badr Brigade militia. "We'll prevent that because that means canceling the constitution and the results of the elections and entering a dark tunnel, which is something we will never allow."