devastated the tombs of the tenth and eleventh of the 12 Imams believed by Shia to have been infallible successors to the prophet Mohammad.

Early reports suggested armed insurgents, possibly in Iraqi police uniforms, entered the shrine and left explosives. US troops and Iraqi police cordoned off the area and began house-to-house searches.

No one was killed in the attack on the mosque in Samarra. However a Sunni cleric was killed, police said, at one of 17 Sunni mosques in Baghdad fired on by militants.

The Samarra attack appears to be a deliberate provocation of Iraq's Shia community, who make up around 55 per cent of the population and comes after two deadly explosions targeting Shia civilians in Baghdad on Monday and Tuesday.

Prime Minister Ibrahim al-Jaafari, a Shia, declared three days of mourning and called for Muslim unity. He said the interim government had sent officials to Samarra.

Mouwafak al-Rubaie, national security advisor, blamed Sunni militants of Ansar al-Sunnah, a group linked to al-Qaeda that has been responsible for a trail of violence in Iraq since the 2003 US-led invasion. Mr Rubaie told Al Arabiya satellite television such militants intended to "to pull Iraq toward civil war".

There has been growing concern among Shia Muslims, in Lebanon and Iran as well as Iraq, at attacks by mainly Sunni insurgents on Shia civilians and religious figures in Iraq.

Ayatollah Sistani, Iraq's most senior Shia cleric, asked people to protest peacefully in their home cities rather than travel to Samarra, which has a majority Sunni population.

Ayatollah Sistani has consistently urged his followers to show restraint and not be goaded by attempt to incite sectarian war.

Jack Straw, Britain's foreign secretary, echoed the call for restraint, calling the attack "a criminal and sacrilegious act" that was "a blatant attempt to ingnite civil strife and disrupt the process of forming a new government in Iraq".

"This is a most shocking outrage against a holy shrine of the Shia community, so all of us have to understand the anger people feel when such defilement of their shrine takes place."

He was confident, however, that Iraqis would continue to demonstrate the "remarkable resiliance and determinination" that they had shown in the past to secure a peaceful and democratic future, as illustrated by high turn-out in the December elections.

The Sunni Endowment, a government body that maintains Sunni shrines, condemned the blast and said it would send a delegation to Samarra to investigate.

The Askariya shrine in Samarra is the tomb of both Imam Ali bin Mohammad, who died in 868AD, and Imam Hassan bin Ali, who died in 872AD and was the father of Imam Mohammad bin Hassan, whom Shia believe went into hiding in 941 and will return one day as 'the Mahdi' to inaugurate a period of just rule on earth before the Day of Judgement.

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