BAGHDAD, Iraq (AP) – The U.S. command announced Saturday that it was sending 3,700 troops to Baghdad to try to quell the sectarian violence sweeping the capital, and a U.S. official said more American soldiers would follow as the military gears up to take the streets from gunmen.

The 172nd Stryker Brigade, which had been due to return home after a year in Iraq, will bring quick-moving, light-armored vehicles to patrol this sprawling city of 6 million people, hoping security forces respond faster to the tit-for-tat killings by Shiite militias and Sunni Arab insurgents.

The U.S. military hopes more armor will intimidate gunmen, who in recent weeks have become more brazen in their attacks.

“This will place our most experienced unit with our most mobile and agile systems in support of our main effort,” said Gen. George W. Casey Jr., the top U.S. commander in Iraq. “This gives us a potentially decisive capability to affect security in Baghdad.”

President Bush said this week that he had decided to send more troops to Baghdad after the surge in reprisal killings began to threaten the unity government of Prime Minister Nouri al-Maliki, which took power May 20.

The wave of violence has dashed administration hopes for substantial reductions in the 127,000-member U.S. mission in Iraq before the November midterm elections.

According to the United Nations, about 6,000 Iraqis were killed in insurgent or sectarian violence in May and June – despite American hopes that the unity government of Shiites, Sunnis and Kurds would win public confidence and ease the security crisis.

The U.S. statement did not say when the Stryker Brigade would move to the capital from its base in Mosul, 225 miles northwest of Baghdad, but the redeployment was expected soon.

A U.S. military official told The Associated Press that more troops will follow the Stryker brigade, normally based at Fort Wainwright, Alaska. The official gave no further details and spoke on condition of anonymity for security reasons.

Pentagon officials have said plans call for adding military police, armored vehicles and tanks to the streets of the capital to work alongside Iraq’s U.S.-trained police and army units. Those units are heavily Shiite, and the presence of Americans is intended to assure Sunnis that the Iraqi forces are not Shiite death squads in uniform.

U.S. and British officials have said Iraqi units, especially the police, have been infiltrated by Shiite militias and have lost the confidence of many Iraqi civilians.

However, the strategy also risks further discrediting Iraqi forces, affecting their morale and making Americans more vulnerable to attack. U.S. casualties have eased in recent months as Americans handed over more security responsibility to the Iraqis and assumed a support role.

But the bitterness of the sectarian conflict and the high stakes at play have proven too much for the Iraqi force in the capital. The surge in attacks also pointed to the failure of al-Maliki’s security plan for Baghdad, unveiled with great fanfare last month.

Sectarian strife worsened after the Feb. 22 bombing of a Shiite shrine in Samarra and threatens to unravel the fabric of Iraqi society.

Last week, U.S. spokesman Maj. Gen. William Caldwell described Baghdad as a “must-win” not only for al-Maliki’s government “but for al-Qaida in Iraq,” which the Americans blame for fanning sectarian hatred.

On Friday, a top Shiite politician allied with al-Maliki said Iraqis – and not Americans – should be given responsibility for security and called for an end to “interference in their work” – an apparent reference to U.S. efforts to curb abuses by the Shiite-led police.

In the Shiite town of Suwayrah, 25 miles south of Baghdad, Mayor Hussein Mohammed al-Ghurabi, said Saturday that more than 500 armed Sunnis had gathered in a nearby village and were firing on his town daily.

Tens of thousands of people have abandoned their homes in religiously mixed neighborhoods, either fleeing abroad or to areas where their sect dominates. They include members of country’s elite – physicians, professors and other professionals.

The Iraqi soccer federation said the country’s national coach, Akram Ahmed Salman, had resigned after receiving a death threat and fled with his family to the relative safety of the Kurdish-ruled north.

The chairman of Iraq’s National Olympic Committee and dozens of other sports officials were abducted during a meeting this month in Baghdad and most remain missing. Jacques Rogge, president of the International Olympic Committee, renewed calls Saturday for their release.

In a bid to curb the violence, U.S. troops have been cracking down on Shiite and Sunni extremist groups in Baghdad and in cities on major transport routes leading to the capital.

U.S. and Iraqi troops detained 25 men suspected of a July 17 attack on a market in Mahmoudiya, the U.S. military said. About 50 people were killed in the attack – mostly Shiites.

American troops clashed Saturday with gunmen of the Mahdi Army militia, loyal to radical Shiite cleric Muqtada al-Sadr, in Diwaniyah, 80 miles south of Baghdad, police said. Seven militiamen were wounded but a local militia leader sought by the Americans escaped, police said.

In other violence:

– The U.S. command said three U.S. Marines were killed Thursday in western Iraq.

– A Sunni cleric from a tribe opposed to al-Qaida in Iraq was killed in Samarra, 60 miles north of Baghdad, police said.

– Gunmen assassinated the western regional commander of the Iraqi Border Protection Force, Brig. Gen. Jawad Hadi al-Selawi, in Karbala, 50 miles south of Baghdad, police said.

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Associated Press correspondent Qais al-Bashir contributed to this report from Baghdad.

AP-ES-07-29-06 1713EDT