Bombings and shootings soared by 40 percent in the Baghdad area in the past week, the US military said Thursday. An American general said extremists were preparing "an all-out assault" on the capital in a decisive battle for the future of Iraq.

Iraqi soldiers take their position at the site of a car bomb attack in which one man was killed and 15 wounded in central Baghdad July 20, 2006. [Reuters]

Iraq's most influential Shiite cleric issued his strongest call yet for an end to Shiite-Sunni bloodletting, urging all Iraqis to wake up to the "danger threatening the future of the country" and stand "side-by-side against it."

US military spokesman Maj. Gen. William Caldwell said there had been an average of 34 attacks a day involving US and Iraqi forces in and around the capital since Friday, up sharply from the daily average of 24 registered between June 14 and July 13.

"We have not witnessed the reduction in violence one would have hoped for in a perfect world," Caldwell told reporters. "The only way we're going to be successful in Baghdad is to get the weapons off the streets."

Caldwell said insurgents were streaming into the capital for "an all-out assault against the Baghdad area."

"Clearly the death squad elements, the terrorist elements, know that Baghdad is a must-win for them," he said. "Whoever wins the Baghdad area, whoever is able to bring peace and security to that area, is going to set the conditions to stabilize this country."

But much of the bloodshed has been carried out by Shiite militias seeking retribution for attacks by Sunnis, including organized insurgents, religious extremists and Sunnis not affiliated with resistance groups but fearful of Shiite gunmen.

The result is a pattern of tit-for-tat vendetta killings which is difficult to stop by military action or political overtures to Sunni insurgent leaders.

With thousands fleeing areas where their sect is in the minority, Iraqis fear Baghdad is being transformed into a Sunni west and a Shiite and Christian east, divided by the Tigris River that flows through the center of the city.

Alarmed by the crisis, Grand Ayatollah Ali Al-Sistani issued a rare statement, saying the time has come for "all those who value the unity and future of this country" to "exert maximum efforts to stop the bloodletting."

Al-Sistani, a longtime voice of moderation, urged Iraqis against "falling into the trap of sectarian and ethnic strife," which he said will only delay the departure of foreign troops.

"I repeat my call today to all Iraqis of different sects and ethnic groups to be aware of the danger threatening the future of the country and stand side-by-side against it," he said.

Caldwell's comments were among the most frank by a senior American military official about the grave crisis facing Prime Minister Nouri al-Maliki's two-month-old national unity government.

US officials have long pointed to relative peace in many of Iraq's 18 provinces, dismissing the insurgency as a problem limited to Baghdad and sparsely populated Sunni Arab areas to the west and north.

However, Baghdad is the country's major transportation hub, the center of political and economic power, and home to more than 20 percent of the population. Its religiously and politically mixed population makes it a natural battleground for control of the country.

"Baghdad is a must-win not only for the prime minister, but for al-Qaida in Iraq," Caldwell said. "Without Baghdad's centralized access to power brokers, Baghdad's large, diverse population, its financial resources, the terrorists elements will lose here in this country."

With the stakes high, al-Maliki last month unveiled a much-heralded 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 kidnappings of Iraqi officials, leading politicians from Shiite and Sunni parties have declared the plan a failure. The United Nations said this week that about 6,000 civilians were killed in May and June, many of them in sectarian violence.

About 50 people were killed Thursday in attacks nationwide, police said. They included a US Marine killed in Anbar province and 12 people who died in a car bombing near Beiji, 155 miles north of Baghdad. Five others were killed by a car bomb in the northern city of Kirkuk.

The crisis in Baghdad raises questions about the ability of Iraq's U.S.-trained police and army to cope with sectarian violence. That in turn casts doubt about the U.S. timetable for handing over security responsibility to the Iraqis in all 18 provinces by the end of next year.

Despite the recent bloodshed, National Security Adviser Mouwafak al-Rubaie said Iraqis will be in charge of security in eight of the 18 provinces before year's end. However, he said the fight against insurgents could last for years.

The government said al-Maliki has dismissed an undisclosed number of security officials for failing to respond to a Monday attack in Mahmoudiya in which at least 51 people were killed. Suspected Sunni gunmen went on a rampage through a market, shooting at shoppers and vendors. Most of the victims were Shiites.

Nevertheless, Caldwell insisted Iraqi forces were "giving their all to bring security to the Baghdadi citizens." He said at least 92 Iraqi police and soldiers had been killed and 444 wounded in fighting in the capital since mid-June.