Fires blazed from the battle-torn streets, after the US military resorted to heavy shelling.

The skies over Fallujah are so crowded with US military aircraft that they are layered in stacks above the city, from low-flying helicopters and swooping attack jets to a jet-powered unmanned spy drones.

No fewer than 20 types of aircraft have been thrown into the fight, including 10 fixed-wing planes, three types of helicopters and seven kinds of unmanned drones.

“We call it the wedding cake. It’s layered all the way up,” said Air Force Lieutenant Colonel David Staven, who leads the ground targeting effort on a US base outside Fallujah.

Much of the air war is being directed by 10 teams of ground controllers, who moved into the city with Army and Marine fighters.

The controllers call down bombing raids or rocket attacks on insurgent positions in the city, said Colonel Staven, who leads the 9th Expeditionary Air Support Operations Squadron.

Sometimes the bombing raids are just blocks ahead of advancing US troops.

But US troops are also heavily involved in house-by-house battles to secure neighbourhoods from north to south.

As night fell, fierce fighting exploded in the heart of the resistance, the Jolan district of north western Fallujah.

“We control about 75 per cent of the city. The more we go in, the more we find the fight is becoming fiercer,” Marine Major PJ Batty said.

“We planned to take this city in 108 hours and we are right on schedule,” he said.

The house-to-house searches have uncovered some grisly finding, among them three Iraqi hostages, handcuffed and tortured, in the basement of buildings in the city.

Major Batty said it is believed to be an operating base for the most ruthless gang of kidnappers in Iraq led by Abu Musab al-Zarqawi.

As the battle continues the body count and list of wounded is mounting.

The US now says 18 servicemen and 5 Iraqi soldiers have been killed, approximately 500 insurgents are dead while 69 US servicemen and 34 Iraqi soldiers have been wounded.

There are no clear figures about how many civilians have been caught in the crossfire.