BAGHDAD, June 13 -- Early morning blasts Wednesday destroyed two minarets at the same Shiite shrine in Samarra where an attack last year demolished the mosque's gilded dome and plunged the country into a wave of deadly sectarian violence.

No one was injured in the 9 a.m. explosions at the revered Askariya shrine in Samarra, about 65 miles north of Baghdad. But officials said it was just the sort of event that could spark a spiral of retaliatory attacks and make it harder to reduce the violence that has brought the addition of thousands of extra U.S. troops stationed at high-profile posts on the streets of Baghdad and elsewhere.

The Feb. 22, 2006, attack on the shrine -- historically known as the Golden Mosque because of its brilliant dome -- was a seminal moment in the four-year Iraq war, sparking a vicious cycle of bloodshed that has never fully stopped. In the 16 months since, thousands of Iraqis -- and perhaps tens of thousands -- have been killed in Sunni-Shiite fighting.

Immediately after the 2006 attack, Shiite death squads accelerated their killings, dumping thousands of mutilated bodies -- most of them Sunni Arabs -- around the capital. More than 100 Sunni mosques were damaged in counter strikes. Tens of thousands of Sunnis and Shiites were driven from their homes in bouts of ethnic cleansing, including in Samarra, which was always predominantly Sunni, but now almost exclusively so.

Fearing a backlash from the latest attack, the Iraqi government imposed an indefinite curfew across Baghdad starting 3 p.m. Wednesday. U.S. military officials said Samarra remained quiet in the hours after the attack.