At least 15 tanks were seen on Wednesday taking up temporary stations one kilometre past the US checkpoint that leads into the city.

Iraqi officials in Falluja confirmed that American troops had asked local authorities to provide them with safe passage through the city.

A Sunni Muslim bastion, Falluja was rocked to its foundations in April by some of the heaviest fighting in Iraq since the US-led invasion began last year.

The last US troop patrol in the city was on 10 May, shortly after they struck an agreement with resistance fighters to end a weeks-long siege and allow Iraqi police and ex-army figures to maintain security.

The tank patrol follows an attack overnight on an Iraqi general charged with imposing security in Falluja.

The raid killed 12 members of General Muhammad Latif's militia and wounded 10 more, though Latif is not believed to have been injured.

Pipeline targeted

Earlier, saboteurs ruptured an oil pipeline linking the Kirkuk oil fields with Iraq's largest fuel refinery at Baiji, 200km north of Baghdad.

Iraqi officials confirmed that the night attack on Wednesday forced a 400-megawatt power station near the refinery to shut down. 
  

The Kirkuk pipeline attack caused
a power station to be shut down

Firemen were still battling on Wednesday to put out the fire on the line.
  
On Sunday, the main oil export artery from the north was also ruptured with sound grenades - causing a sharp rise in world oil prices.

Northern Iraq's pipeline, which takes crude from the Kirkuk oilfields to Turkey's Mediterranean terminal of Ceyhan, has not been at full capacity use since the US-led invasion last year.

Convoy ambushed
  
Elsewhere, resistance fighters raked a US military convoy on Wednesday. One of the trucks exploded, an Iraqi witness and a US soldier said.
  
The attack was launched on a road in the northwestern Baghdad neighbourhood of al-Khadra.
  
A crowd gathered and cursed the firemen dousing the blaze and demanding they let it burn.
  
"One soldier ran away from the truck and got into another vehicle," said Burhan al-Din Husain, who was getting his car fixed at a mechanics shop when the speeding car shot up the convoy.
  
He said it happened between 10:30 and 11:00 GMT. A US soldier said the truck caught fire when a a Molotov cocktail was thrown at it.

The stretch of road is the site of frequent attacks on occupation troops.