Christians gathered for Sunday mass after coordinated bomb attacks on five Baghdad churches sent chills through the rapidly shrinking religious community, on edge over the nation's rampant lawlessness and rising tide of Islamic extremism.

A brief statement from the US military announced that two reconnaissance helicopters had crashed in southwest Baghdad on Saturday evening, killing two soldiers and wounding two others.

The incident was under investigation and US officers refused to speculate on whether the helicopters had been shot down.

The deaths followed the killing of four US troops in a series of bombings in various parts of the country reported Saturday.

The spate of attacks during Ramadan evoked memories of the holiday last year when insurgents stunned Iraq with their most serious campaign until that time, leaving 82 US soldiers dead.

On the eve of the holy month on Thursday, two bomb attacks hit the previously impregnable fortress-like Green Zone in Baghdad, home to the Iraqi government and the US embassy, leaving five people dead, four of them US civilians.

More than 1,000 American and Iraqi troops sealed off the rebel hub of Fallujah for a third straight day in their hunt for Jordanian fighter Abu Mussab al-Zarqawi, the country's most wanted man blamed for some of the deadliest attacks since last year's invasion.

Late Saturday, US warplanes struck Fallujah in what the US army billed a raid "against an armed al-Zarqawi terrorist checkpoint" controlling the flow of people in and out of the city, which the Iraqis and Americans want tamed before January elections.

Fallujah general hospital reported three dead.

Local cleric Sheikh Abdul Hamid Jadu indicated that a delegation from the city was ready to return to the bargaining table with the government, but conditioned this on a halt to US air strikes and the release of fellow negotiator Sheikh Khaled Hamoud.

The team severed dialogue with the Iraqi government Thursday amid an escalation of the air strikes and a threat by Iraqi Prime Minister Iyad Allawi that Fallujah must surrender Zarqawi or face invasion.

Rebels have unleashed a string of car bombings since Friday, killing three US soldiers and their translator by the porous Syrian border, considered a for weapons, cash and professional Jihadists. A soldier in the northern battleground city of Mosul also died Friday in a car bomb attack.

A US marine was wounded Saturday when a suicide car bomb detonated alongside a tank in the town of Khalidiyah between Fallujah and Ramadi, the two cities viewed as the heart and soul of the resistance.

The deaths brought to 1,089 the number of US soldiers killed in Iraq since the US-led invasion in March 2003, according to an official Pentagon tally.

A statement attributed to Zarqawi's group on an Islamist website claimed Saturday it had beheaded 11 members of the Iraqi police and national guard on central Baghdad's Haifa street, the scene of frequent clashes between the rebels and the US and Iraqi security forces.

Iraq's national intelligence director, General Mohamed Shahwani, says Haifa street serves as Zarqawi's headquarters in the capital.

Iraq's tiny Christian community was in shock Sunday after apparently coordinated predawn blasts at five Baghdad churches the previous day. Worshippers planned to gather for mass at the blackened Saint George Church in the city's Karrada district.

The White House condemned the church attacks.

"We denounce any and all violence against the Iraqi people," spokesman Trent Duffy said, but added: "The insurgent killers are sealing their own fate with these attacks because the vast majority of Iraqis have had enough."

Christians, who make up just three percent of the population in the Muslim majority country, were victims of a similar attack at the start of August in which 10 people died and 50 were injured.

After that attack, the government said 40,000 Christians fled the country, further shrinking the community of 700,000 people that traces its roots in the land of Mesopotamia back to biblical times.

In other unrest, an Iraqi national guard cycling to work was shot dead by gunmen in Khallis, a town just north of the flashpoint city of Baquba, a fellow soldier said. The water supply at Baquba general hospital was cut off Sunday after a rocket attack overnight, medical sources said.

In a possible boost for US-led forces, the South Korean government is to seek parliamentary approve to extend the deployment of its troops in Iraq by a year, news reports said.

The mission, involving up to 3,600 South Korean troops in Arbil, a Kurdish-controlled town in northern Iraq, was due to expire at the end of this year.