By Peter Graff

BAGHDAD, Oct 23 (Reuters) - U.S. forces declared an area once known as the "triangle of death" safe enough for Iraqi troops to take charge on Thursday, handing over responsibility for security in Babil province to Iraqi forces.

The province south of Baghdad is the 12th of Iraq's 18 provinces in which primary responsibility for security has been given to Iraqi forces.

With violence at four-year lows, only the capital Baghdad, four ethnically and religiously mixed northern provinces and Wasit province along the Iranian border still require day-to-day U.S. patrols of Iraqi streets.

Iraq's National Security adviser Mowaffaq al-Rubaie said at a handover ceremony in the provincial capital Hilla that Iraqis will also take control of Wasit province within days.

"This is proof that our military forces have reached self-sufficiency, and can now be depended upon to preserve internal security," Rubaie said.

Lieutenant-General Lloyd Austin, commander of U.S. combat forces in Iraq, called the event a "milestone for Iraq in maturing as a sovereign and democratic nation."

"Just a year ago this province was experiencing well over 20 attacks per week. And today attacks are down well over 80 percent. This is truly remarkable," he said.

Iraqi troops, police and firemen then marched past to the sound of a brass band.

Babil, a vast province named for the ruins of ancient Babylon, includes rural Sunni Arab areas dotted with date palm groves along the Euphrates south of Baghdad, which U.S. forces dubbed the "triangle of death" in years after the 2003 invasion.

The triangle was a heartland of the Sunni insurgency against U.S. forces and the Shi'ite-led government in Baghdad until last year, when many Sunni Arabs joined U.S.-funded patrols known as "Awakening" groups.

Babil's cities have also seen uprisings by followers of anti-American Shi'ite cleric Moqtada al-Sadr as recently as April this year. But they have been far quieter in recent months as Sadr's followers have held to a ceasefire.

Violence across Iraq has fallen over the past year to four-year lows, but militants are still able to carry out frequent car bomb and suicide bomb attacks. Suicide bombings are a signature tactic of al Qaeda Sunni Islamist militants.

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