Insurgents blew up two minarets of a most revered Shiite shrine in Samarra City, some 120 km north of Baghdad, on Wednesday morning, raising fears over escalation of the sectarian violence between Shiites and Sunnis in Iraq.

Unidentified insurgents destroyed with explosives at about 9 a.m. (0500 GMT) two minarets of the Imam Ali al-Hadi shrine, whose golden dome was destroyed in February 2006 that triggered a wave of retaliatory sectarian violence, a local policeman told Xinhua by telephone.

Earlier, several mortar rounds landed near the two shrines, but no casualties and damages were reported immediately, the source said.

Shortly after the attack, hundreds of local residents in Samarra City took to streets to protest before a curfew was imposed.

It was unclear exactly how the minarets had been destroyed, because the shrine is heavily guarded by the police.

Local media reported that insurgents clashed with the police for several hours and some insurgents managed to plant bombs near the minarets and blew them up. However, the U.S. military said the policemen guarding the mosque were detained and evidence show that the attack was an inside job.

Both U.S. and Iraqi government accused al-Qaida militants of launching the attack.

U.S. Ambassador Ryan C. Crocker, and the U.S. military top commander General David Petraeus, said in a joint statement that "this is a deliberate attempt by al-Qaida to sow dissent and inflame sectarian strife among the people of Iraq to obstruct the peaceful political and economic development of a democratic Iraq."

In a televised speech, Iraqi Prime Minister Nuri al-Maliki also blamed al-Qaida for the attack and called on Iraqis to unite.

Fearing further violence, the prime minister imposed a curfew in the capital starting from 3 p.m. until further notice.

Iraq's top Shiite religious leader Grand Ayatollah Ali al-Sistani, urged Shiites not to carry out reprisal attacks against Sunnis.

Iraq's radical Shiite cleric Muqtada al-Sadr called for three days of mourning and calm and peaceful demonstrations for the destruction of the two minarets.

However, Sadr's political bloc with 30 lawmakers decided to suspend its membership in parliament following the attack "until the Iraqi government takes strong measures to reconstruct all the Sunni and Shiite shrines, especially the Shiite shrine in Samarra."

On Feb. 22, 2006, the Shiite shrine of Ali al-Hadi, also called the Golden Mosque, was attacked by a bomb, with its 100-year-oldgolden dome badly damaged.

The shrine of Ali al-Hadi is one of the four most revered Shiite shrines in Iraq. It contains tombs of Ali al-Hadi, who died in 868 A.D., and Hisson Hassan al-Askari who died in 874 A.D.

The two are the 10th and 11th of the Shiite's twelve most revered Imams. Shiite pilgrims visited the shrine from all over the world.

Last year's destruction of the golden-domed mosque enraged Shiites, particularly Shiite militia that have been on a rampage of revenge killing ever since. Sunnis have fought back with equal vengeance.

Thousands of Iraqi citizens have been killed just for their sects, which pushes the war-torn country on the brink of civil war.

The latest attack immediately raised fears of a new round of tit-for-tat attacks between rival sects, which probably will draw the country to a full-scale civil war.

The attack also came at a very critical time for Iraq. A major U.S.-Iraqi security crackdown in Baghdad, which aims at buying space and time for Iraqi politicians to meet a series of benchmarks, has no tangible progress after nearly four months. On the political front, there are also no sign of progress.