Guerilla resistance is stiffening, in contrast to the crushing of Iraq's regular army by US-led forces seven months ago -- a rout partly caused by Saddam's delusion the invasion was only a feint, his former deputy prime minister was quoted on Monday as saying.

Eighteen Americans died in guerrilla attacks in Iraq on Sunday, including 15 soldiers killed when insurgents shot down their giant Chinook helicopter near the flashpoint town of Falluja near Baghdad. A further 21 were wounded.

US troops and Humvees guarded the wreckage yesterday.

Nearby, gleeful villagers from old men to children celebrated the shooting, calling it the perfect present to mark the Muslim holy month of Ramadan. The "Sunni triangle" region, where the helicopter was downed, is a hotbed of anti-US anger.

"We usually celebrate Ramadan at the end of the month. Now we are celebrating in the beginning after these infidel Americans were shot down," said taxi driver Abdullah Hissein.

More US helicopters clattered overhead. "Now we want to take them down as well," the driver said.

Earlier, 18 of the wounded soldiers arrived at the US Ramstein Air Base in Germany. The military said some would be treated at the nearby Landstuhl Hospital.

The triumphal postwar glow in which President George W. Bush taunted Iraqi militants with the challenge "bring them on" has faded to a grim determination against an ever more lethal resistance which has forced most foreign aid workers to leave.

Sunday was the second deadliest day overall for Americans in Iraq since the war started on March 20, after 28 soldiers were killed in various attacks on March 23. At least 250 US soldiers have been killed by hostile fire since the invasion.

The number of Iraqis killed since March is in the thousands and steadily climbing.

On Sunday night near Balad, north of Baghdad, US forces fired on a pickup truck, killing six Iraqis, residents said. US soldiers said the vehicle was suspected of carrying insurgents and that the incident was being investigated.

Locals said the group had been returning from prayers.

The US-led administration said in a statement that Mustafa Zaidan al-Khaleefa, head of Baghdad's Karkh Neighborhood Council, was killed on Sunday evening while walking near his home. Two gunmen shot him as they drove by, it said.

Loud blast near US HQ in Baghdad

Reuters, Baghdad

A large explosion echoed across Baghdad on last night, followed by the sound of mortar fire near the headquarters of the US-led administration.

Reuters witnesses heard a loud blast followed by the sound of several mortar rounds exploding, coming from the west bank of the Tigris river, where the US-led administration occupies a large palace complex previously used by Saddam Hussein.

There was no immediate comment from the US military.