Perspective: On the trail of Iraq's deadly mortar squads

Above: Sandbagged windows provide extra protection at a building inside the U.S. Embassy complex in Baghdad's 'Green Zone'.

In the still of Baghdad's hot midnight, among thick palm groves that line the Diala river to the city's south, the sound of helicopters strafing the tree line a few hundred metres distant seems sickeningly close.

The Apache gunships are invisible except for the bright commas of gunfire that appear for the briefest of moments. The noise catches up, an ugly bass drilling sound that jars even the US soldiers in their vehicles and uniforms marked to be visible from the air.

It is not quite random firing into the farmland and groves. It is a calling card to the mortar teams who use these secluded spots to fire into the distant Green Zone. An hour or so earlier, it was artillery firing out of one of the forward operating bases, the explosions echoing among the tall, spare trees.

The soldiers call it 'area denial'. When a shell overshoots its target by 600 metres, it scatters razor-sharp fragments across the roofs of the nearby houses.

This weekend the firing has an extra urgency. Following tips from Iraqi civilians, the hunt is on for a new and deadlier kind of rocket, modelled on one of those used by Hizbollah, the al-Sharooq.

In the pictures the soldiers have been given it looks like an artillery shell with jet boosters, fired out of its own launcher. It can fly six miles with an accuracy far greater than any of the rockets or mortars deployed so far. It can penetrate armour and the hardened shelters of the Green Zone - a new and dangerous escalation in the war in Iraq. Tonight, sources say, it will be fired for the first time.

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