A US military transport helicopter crashed north of Baghdad today, the fifth loss in a fortnight, raising fears that insurgents have found a new weapon to counter US air superiority.

The twin-rotor CH46 Sea Knight — a marine version of the Chinook, which can carry up to 25 fully equipped troops — crashed 20 miles (32km) northwest of Baghdad, a few days after a US spokesman conceded that the military was changing its tactics to counter the threat.

US helicopters, already a common sight over Baghdad, have been flying in greater numbers in recent days as thousands of US reinforcements begin arriving in and around Baghdad for President Bush’s troop “surge” to implement the increased security in Baghdad, now in its early stages.

The relatively few main helicopter landing pads are well known to the insurgents, who can easily track the aircraft’s final approaches and departures. Chinooks are instantly identifiable from a distance because of their twin-rotor profile and much louder sound.

Jaish al-Mujahidin (Mujahidin Army) was one of the insurgent groups which claimed responsibility today, saying the helicopter was downed near Garmah.

Abu Outba, a Jaish commander, said: “It was hit by one of our battalions which specialises in hunting down American helicopters.” He claimed they used an Russian surface-to-air missile that “ignores the flares which the helicopter throw out to avoid missiles.”

Major General William Caldwell, a US military spokesman would not confirm if anyone had been killed or wounded, or if the helicopter had been shot down.

“A quick reaction force is on site and the investigation is going on as we speak,” he said in Baghdad. “It would probably be inappropriate for me to talk about whether or not there are or are not casualties.”

However, witnesses spoke of rocket fire from the ground, and of insurgents celebrating. “Early in the morning fighters from the resistance managed to sneak to the area banned by the Americans east of Garmah, near farms and warehouses, and they managed to set a trap inside these farms,” said Ali Thamir, a local resident.

“The helicopter was heading to Habaniya base west of Fallujah but it was hit by a missile and we could see it when it was blown up and how its parts flew through the air.

“Now there are a large number of helicopters flying over the town in all directions. We also saw four fighters from the resistance on the back of a pickup truck celebrating and driving around Garmah, broadcasting the Koran on loudspeakers.

Last week a US Army helicopter crashed amid gunfire near Baghdad, killing two. Three others helicopters have been downed since January 20, including one during a firefight with a Shia cult in Najaf, and another civilian spotter helicopter, killing five security contractors whose bodies were found with bullets in their head, execution-style.

At the time Major General Caldwell said that although investigations were ongoing, “it does appear that they were all the result of some kind of anti-Iraqi ground fire that did bring those helicopters down.” He said the US military was “making adjustments in tactics, techniques and procedures and are making the appropriate changes”.

He confirmed today that Prime Minister Nouri al-Maliki’s Iraqi-led “Operation Imposing Justice” to restore security to Baghdad “is ongoing as we speak”. However, he conceded that it was not fully operational because the Iraqi security forces were still moving “additional forces” into the city and were “still currently today working on some of the economic aspects of this plan.” Previous operations have founded on promised Iraqi troops not being made available.

But Major General Caldwell insisted: “The implementation of the Prime Minister’s plan has already begun and will be fully implemented at a later date, having all the parts and pieces that he wants.”

Only 24 hours earlier Mr al-Maliki had instructed his commanders to hasten their preparations, amid criticism that it was being unrolled too slowly, allowing suicide bombers to inflict hundreds more casualties in recent days.

“The operations will unite us and we will take action soon, God willing, even though I have the feeling that we have been late, and this delay has started to give a negative message.”

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