SAMARRA (Iraq), June 13: Suspected Al Qaeda militants blew up the two minarets of a revered shrine in the Iraqi town of Samarra on Wednesday, 15 months after a first bombing sparked brutal sectarian killings.

The destruction of the gold-covered minarets of the Al-Askari mosque in the largely Sunni town north of Baghdad raised fears of a resurgence in inter-communal violence, with indefinite curfews imposed in Samarra and Baghdad.

Hours later, four mosques were attacked in apparent retaliation, police said, while hundreds of people marched in protest in the cities of Karbala, Najaf, Nasiriyah, Samawa and Basra.

Amid widespread appeals for unity and calm, top US officials in Iraq blamed the bombing on Al Qaeda, while Iraqi Prime Minister Nuri al-Maliki ordered a probe into the attack.

Interior ministry spokesman Brigadier General Abdul Karim Khalaf said “a group of terrorists was arrested and being interrogated,” but did not clarify whether they were suspected of bombing the shrine.

In Washington, the White House promised an all-out effort to prevent the attack from sparking a surge in sectarian violence.

“What we're hoping is that there won't be a new wave” of clashes between Iraq's Shias and Sunnis, said spokesman Tony Snow.

“It is clear that somebody is trying, once again, to use this holy site as a venue for trying, literally, to blow up Iraqi democracy,” said Snow, who told reporters the strike “has all the marks of Al Qaeda.” The US military said the Samarra minarets were destroyed at around 9:00 am (0500 GMT), adding that no casualties were reported.“I urge everybody not to give a chance to terrorists and stand united in confronting those who ignite sectarian passions,” Maliki said on state television.

He later visited the bombed shrine with the interior and defence ministers.

US national security spokesman Gordon Johndroe said that “now is the time for all Iraqis to come together to defeat the extremists who offer nothing but death and destruction.” British Foreign Secretary Margaret Beckett called the bombing “blatant and crude,” and urged increased efforts of national reconciliation.

The attack was a grim echo of the Feb 22, 2006 Al Qaeda bombing of the shrine, which triggered brutal Shiite-Sunni bloodletting that continues to this day and has left tens of thousands dead.

That attack destroyed the golden dome of one of the world's holiest Shia shrines.

A witness said the two minarets fell within minutes of each other.

“I was near the shrine when I heard big explosions that sent a thick cloud of dust in the sky covering the entire area,” he said.

“I quickly ran to the street from where I could see the shrine clearly. I saw one of the minarets was down. Seven minutes later as I was watching the shrine, another explosion occurred and the second minaret came crumbling down.” The area had been heavily fortified since last year's attack, but the witness, who spoke on condition of anonymity, said there had been a row on Tuesday between old and new security teams supposed to protect the shrine.

Iraq's Shia cleric Moqtada al-Sadr called for three days of mourning and blamed the US-led “occupation” for the attack, as his bloc boycotted parliament in protest.

“Let the Iraqi people be aware that no Sunni has attacked the shrine, but it is the occupation's hidden hand which wants bad things to happen to us,” he said in a statement.

Iraq's revered cleric Grand Ayatollah Ali al-Sistani called the attack “heinous” and urged “the faithful to be patient and to refrain from taking revenge against innocents and the holy sites of others.” He criticised the government for what he called its past inactions and said he hoped the shrine would soon be rebuilt.

US officials blamed Al Qaeda for what they called “an act of desperation.” “This brutal action on one of Iraq's holiest shrines is a deliberate attempt by Al Qaeda to sow dissent and inflame sectarian strife,” said a joint statement from Ambassador Ryan Crocker and the head of US forces in Iraq, Gen David Petraeus.—AFP