The statement by Grand Ayatollah Ali Al-Sistani came as U.S. military officials reported a 40 percent increase in the daily average of attacks in the Baghdad area.
U.S. spokesman Maj. Gen. William Caldwell said there has been an average of 34 attacks a day against U.S. and Iraqi forces in the capital over the past five days. The daily average for the period June 14 until July 13 was 24 a day, he said.
"We have not witnessed the reduction in violence one would have hoped for in a perfect world," U.S. spokesman Maj. Gen. William Caldwell said at a news briefing Thursday. "The only way we're going to be successful in Baghdad is to get the weapons off the streets."
Caldwell said militias and death squads have responded to Prime Minister Nouri al-Maliki's call for a crackdown by intensifying attacks to derail Iraq's new unity government.
Last month, al-Maliki announced a security plan for Baghdad, including up to 50,000 police and soldiers on the streets, more checkpoints, and raids in neighborhoods where violence is high. But with surging attacks in the capital, including the kidnapping of high-ranking Iraqi officials, leading politicians from Shiite and Sunni parties have declared the plan a failure.
The government said Thursday that al-Maliki had dismissed security officials for failing to respond to a Monday attack south of Baghdad in which at least 51 people were killed. Suspected Sunni gunmen went on a rampage through a market in Mahmoudiya, shooting at shoppers and vendors. Most of the victims were Shiites.
Al-Maliki's office said an undisclosed number of security officials would be replaced and that teams would be sent to examine a water shortage that has led to public discontent.
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Shiite politicians complained that local police and soldiers failed to respond to the attack until the gunmen fled.
Still, National Security Adviser Mouwafak al-Rubaie said Iraqis will be in charge of security in eight of the nation's 18 provinces before year's end. However, he said the fight against insurgents could last for years.
Al-Rubaie's comments came a week after British and Australian forces handed over security for the relatively peaceful southern province of Muthanna to Iraqi forces in the first such transfer.
"There is a detailed plan for the withdrawal of multinational forces from provinces and it started in Muthanna," al-Rubaie said. "It will be followed by other provinces like Najaf, Karbala, Maysan, the three Kurdish provinces, then Wasit."
Officials also said Thursday that four more people abducted last weekend during an Olympic Committee meeting had been released, bringing the number freed to 10. But dozens are still missing, including National Olympic Committee chairman Ahmed al-Hijiya.
Until now, al-Sistani has been credited with restraining the majority Shiite community from widespread retaliation against Sunnis in the face of horrific attacks on Shiite civilians by al Qaeda in Iraq and other Sunni religious extremists.
But the ability of the aged cleric to continue holding back Shiite militias and others has been called into question as attacks increase. The United Nations said this week that about 6,000 civilians had been killed in May and June, many of them in sectarian violence.
Alarmed by the rising pattern of tit-for-tat violence over the past five months, al-Sistani urged religious and community leaders to "exert maximum efforts to stop the bloodletting." He warned that the ongoing violence will only prolong the presence of U.S.-led forces in Iraq.
The cleric said the Feb. 22 bombing of a Shiite shrine in Samarra had unleashed a "blind violence" that was sweeping the country. Unless the violence stops, he said, it "will harm the unity of the people and block their hopes of liberation and independence for a long time."