Deadly attacks on Shiite shrines during one of the sect's holiest days have reinforced fears of civil conflict in Iraq, a nation whose ethnic and religious fabric have looked increasingly fragile since Saddam Hussein's ouster 10 months ago.

Iraqi clerics and politicians, Shiite (search) and Sunni (search) alike, insisted that Iraqis were united in the face of what they said were Al Qaeda attempts to spark sectarian war. But the fury, some directed against the United States, within a religious community defined in part by enduring centuries of persecution may prove too much to contain.

Repeated and somewhat desperate calls for calm by members of Iraq's U.S.-backed Governing Council during a hurriedly convened news conference suggested a genuine concern that the country could plunge into civil strife after dozens of devastating terror attacks that have claimed hundreds of lives in recent months.

The attacks have struck deep at the heart of a nation whose viability as an entity has on occasion been called into question.

"I draw attention to the necessity of keeping calm and patient and to adhere to national unity, in order to stop the enemies who want evil to befall the nation," a Governing Council statement read by Sunni member Adnan Pachachi (search) said. Grand Ayatollah Ali al-Husseini al-Sistani (search), the most influential Shiite cleric in Iraq, made a similar appeal for unity in a statement.

Tuesday's attacks were all the more painful to Shiites because they took place on a day when they mourned the 7th century death in battle of their most beloved saint, Imam Hussein (search), the grandson of Islam's prophet Muhammad.

The brutality of the attacks left many Shiites shocked and anguished. They blamed militant Sunnis, Americans, Jews — a word often used by Arabs to refer to Israel — and Wahhabis (search), followers of a militant Sunni sect whose adherents are mainly in Saudi Arabia.

Grand Ayatollah Hadi al-Mudaressi, the most senior Shiite cleric in Karbala, said coalition troops "did nothing" to protect the hundreds of thousands of visitors in the city. Governing Council member and Shiite leader Abdel-Aziz al-Hakim also blamed U.S.-led occupation authorities and repeated his longstanding demand that Iraqis are allowed to take full responsibility for security.

The loss of Shiite goodwill would prove costly to the United States during a U.S. election year in which the Bush administration needs to show policy successes in Iraq.

Unlike their Sunni Arab compatriots, Shiites have refrained from attacking U.S. occupation forces despite their misgivings about Washington's intentions. Many Shiites, however, say they are prepared to wage holy war against the Americans if their religious leaders sanction it.

Since the 1950s, tensions between Iraq's rival clans, tribes and sects had been prevented from boiling over into open conflict by a succession of authoritarian rulers, including Saddam. His 23-year rule suppressed any sign of sectarianism or religious fundamentalism. The fall of his regime in April left a huge power vacuum that neither the United States nor the Iraqi interim administration have filled.

Rival religious and ethnic groups, meanwhile, are jockeying for position before the June 30 transfer of power to Iraqis. That has deepened the fault lines separating a long-suppressed Shiite majority and a Sunni Arab minority bristling at losing its dominant position. A large Kurdish community yearns for self-rule, bringing it into conflict with Sunni and Shiite Arabs who see it as a prelude to the nation's dismemberment

Juan R. Cole, a University of Michigan professor and author on Iraqi Shiites, doubts fullscale civil war would break out in Iraq because the restraint shown so far by mainstream Shiite and Sunni clerics.

But he said smallscale urban violence between rival sects "is a worst case and plausible scenario."

Members of the Governing Council who spoke to journalists blamed Al Qaeda for the attacks, a charge that's apparently also designed to prevent Shiites from acting on their suspicion that Sunnis were behind them. U.S. Brig. Gen. Mark Kimmitt, deputy chief of operations in Iraq, said he believed a mixture of Iraqis and non-Iraqis carried out the attacks.

The attacks suggested that a blueprint for terror in Iraq found in an alleged letter addressed to the terror network's leadership was already being implemented. The letter by suspected Al Qaeda operative Abu Musab al-Zarqawi, a Jordanian believed in Iraq, proposed attacking Shiite religious sites to draw them into a civil war against Sunni Arabs.

The letter was intercepted by the United States in January and publicized last month.

The Governing Council members made repeated references to the letter, vowing that Iraqis will remain united to foil the plan.

"We resolutely respond to that letter by saying that the sectarian sedition and civil war al-Zarqawi wants will never happen," said Mouwafak al-Rubaie, a Shiite council member.

Iraqis routinely dismiss talk of a civil war, pointing to examples — common in cities but rare in rural areas — of Shiites, Sunnis and Kurds living happily as members of a single family due to intermarriage.

In symbolic gestures, mosque imams in the hardline Sunni cities of Ramadi and Fallujah appealed Tuesday for blood donations to help victims of the attacks on the Shiite shrines.

But such words and gestures may not provide much comfort to the relatives and friends of the victims or restrain Shiite radicals who are likely to seek revenge.