The deaths of more than 100 people in violence between Iraqi security forces and Sunni Arab protesters and their supporters have raised fears of a return to all-out sectarian conflict.
The trouble began on Tuesday when security forces moved into an area near the northern town of Hawijah where Sunnis had been holding protests since January, sparking clashes in which 53 people died.
That fighting set off a wave of revenge attacks that hit five different Sunni-majority provinces, killing dozens more people, and which saw gunmen take control of the town of Sulaiman Bek.
The violence is the deadliest so far linked to demonstrations that erupted in Sunni areas of the Shiite-majority country more than four months ago.
The Sunni protesters have called for the resignation of Shiite Prime Minister Nuri al-Maliki and railed against the alleged targeting of their community by the authorities.
"This is the deepest and most dangerous crisis... since 1921," former national security adviser Mowaffak al-Rubaie said, referring to the year in which the state of Iraq was established.
He warned that the current situation "could lead to a sectarian conflict, and then division."
Sectarian violence, including bombings and death squad murders that peaked in 2006 and 2007, claimed tens of thousands of lives.
The security situation has since improved markedly, but sectarian tensions remain.
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On Wednesday, Abdulghafur al-Samarraie and Saleh al-Haidari, top clerics who respectively head the Sunni and Shiite religious endowments, held a joint news conference in which they warned against sectarian strife.
Samarraie said there were "malicious plans... with the goal of taking the country towards sectarian conflict", and that he and Haidari agreed "to move quickly to extinguish the strife and stop the conspiracy".
US State Department spokesman Patrick Ventrell told reporters that Washington condemns the violence in Iraq and that "there's no place for sectarian conflict in a democratic state."
An earlier statement from the US embassy said that "US officials have been in contact with senior Iraqi leaders to help defuse political and sectarian tensions."
John Drake, an Iraq specialist with risk consulting firm AKE Group, said the high death toll from security force action in recent days highlighted shortcomings in the government's approach to protest.
"I think the government response indicates that it has a long way to go in terms of its policies for dealing with protest movements in the country," said John Drake, an Iraq specialist with risk consulting firm AKE Group.
"The use of force so readily, including firearms, at protest camps and the bombing of settlements where militants are believed to be sheltering, is going to bring a very high risk of collateral damage," Drake said.
"An 'all-out' sectarian conflict is still unlikely," he said.
"But the fact that this is a predominantly Shiite government and it's predominantly Shiite security forces opening fire on predominantly Sunni individuals (civilians or militants) is going to have an impact on sectarian relations and could prompt a rise in sectarian violence as a result."