Roadside bomb attacks on American troops in Iraq reached an all-time high last month, accounting for more than one third of all combat deaths.
The increase in the number of casualties caused by the sophisticated explosive devices comes at the height of the "surge" of United States forces which, the Pentagon claims, is broadly a success.
Washington and London have blamed the Iranian regime of supplying the devices which have been used with lethal effect against American and British troops.
Although the Coalition forces had claimed a number of successes in discovering caches of the bombs, the number of attacks last month - 99 - shows the insurgency has had no problem in obtaining supplies.
US forces have been focusing their operations on Sunni militants and, in particular, al Qaeda in Iraq. One of the aims of the "surge" was to combat Shiite militias which, often in collusion with Government forces, have been running death squads.
But the use of the roadside devices show that the threat from the Shiites, with many of the groups sponsored by Tehran, have not diminished despite numerous American missions.
Lieutenant-General Raymond Odiarno, deputy US commander in Iraq, said last month was an all-time high for attacks using the devices and that Shia militants were responsible for 73 per cent of the attacks which killed or wounded US troops in Baghdad last month.