The threat of civil war lurched closer in Iraq Saturday as a suicide bomb killed four people and Iraq's Shiite prime minister faced criticism as he prepared for a summit with U.S. President George W. Bush.

Iraq's sectarian violence will spread throughout the Mideast unless the international community stops supporting Prime Minister Nouri al-Maliki's government, a prominent Sunni religious leader warned Saturday.

"I call on the Arab states, the Arab League and the United Nations to stop this government and withdraw its support from it," said Sheik Harith al-Dhari, who heads the Association of Muslim Scholars. "Otherwise, the disaster will occur and the turmoil will happen in Iraq and other countries.''

Iraq's Interior Ministry issued an arrest warrant for al-Dhari last week, saying he was responsible for inciting violence and terrorism.

Politicians loyal to radical anti-American Shiite cleric Muqtada al-Sadr are threatening to boycott parliament if al-Maliki meets Bush at the summit in Jordan on Wednesday and Thursday.

The political bloc, known as Sadrists, is a mainstay of support for al-Maliki. The White House has said the summit hasn't been cancelled.

Because U.S. forces failed to provide security, they are to blame for Thursday's deadly attack on Sadr City, claimed Sadrist legislator Qusai Abdul-Wahab.

Insurgents suspected to be Sunni killed 215 people in the attack on the Shiite slum, as mortars and five car bombs were deployed in the deadliest attack so far in the war.

A suicide car bomber attacked a checkpoint near Fallujah on Saturday, killing a U.S. soldier and three Iraqi civilians. Nine civilians and an American service member were wounded, the coalition said.

Two Iraqi children and one adult were killed, demonstrating that the Sunni-led insurgency in Anbar province continues to show a "complete lack of concern'' for civilians, said coalition spokesman marine Lt.-Col. Bryan Salas.

A U.S. marine also died Saturday from wounds received in a fight in Anbar province on Friday.

Insurgent tactics and training improving

Insurgents are beginning to change tactics, military spokesmen say. In addition to bombs and shelling, they are beginning to directly engage coalition troops.

At least 72 insurgents and two American officers were killed in more than 40 hours of fighting during a pitched battle last week in Turki, in Diyala Province, near the border with Iran. U.S. officers said that a platoon deployed by insurgents displayed unexpectedly good military discipline.

The insurgent units are believed to be training in camps east of Baghdad run by militant Sunnis connected to Al Qaeda in Mesopotamia, U.S. commanders said this week.

Colonel Poppas told The New York Times that insurgents established the camps after U.S. forces left the area during the fall of 2005.

Sunni Arab militants in the area there follow the fundamentalist Wahhabi strain of Islam and are believed to be commanded by Abu Abdul Rahman, an Iraqi-Canadian who married a woman from Turki and moved there from Canada in 1995, Poppas said.

A classified United States government report obtained by The New York Times said that the insurgency is flush with cash, raising tens of millions of dollars each year from kidnapping, counterfeiting, corrupt charities, oil smuggling, and other crimes.

The report estimated that of US$70 million to US$200 million being raised each year from crime, US$25 million to US$100 million is coming from oil smuggling. Kidnappings are estimated to be raising US$36 million a year, the Times reported.

Bloodshed in Diyala

As U.S. Vice-President Dick Cheney arrived in Saudi Arabia Saturday to meet with King Abdullah in an apparent bid to try and obtain royal support for U.S. efforts to quell bloodshed in Iraq, sectarian violence shifted to Diyala province north of Baghdad.

Police said that gunmen forced their way into two Shiite homes and killed 21 men in front of their families. In fights in the same region, U.S. and Iraqi forces killed 58 insurgents.

The village was too remote for police to collect the bodies and take them to a morgue until Saturday. Police said it is unknown whether the attack was a tribal dispute or another example of sectarian hatred.

A 24-hour curfew was imposed in Baghdad, allowing people to leave home on Sunday, but without vehicles, which are banned for another day.

Police said that they killed 36 insurgents and wounded dozens of others in clashes elsewhere in Diyala Saturday.

U.S. and Iraqi forces killed 22 insurgents and a civilian in raids north of Baghdad Saturday, and destroyed a factory churning out roadside bombs, the military said.

According to police and witnesses, insurgents bombed and burned four mosques and torched several homes in Baghdad's mostly Shiite neighbourhood of Hurriyah Friday, killing as many as 25 Sunnis.

The U.S. military said Saturday that Iraqi soldiers who secured the town found only one burned mosque, however. They were unable to confirm accounts that six Sunni Arabs were dragged from Friday prayers and burned to death.

On Friday, al-Sadr, the anti-American Shiite cleric, had urged Sunni leader al-Dhari to issue a religious edict condemning Sunni attacks on Shiites.

Al-Dhari told a news conference Saturday that his Association of Muslim Scholars has consistently condemned the killing of Iraqi Muslims and attacks on homes and mosques.

In a bid to assert its role as a powerbroker in Iraq, Iran also planned a summit Saturday, inviting the presidents of Iraq and Syria. Syria did not respond. Iraq's president said he could not head to the summit before Sunday, since the international airport was closed for commercial flights after recent attacks on Sadr City.

With files from The Associated Press