THE US Army is having to add extra armour to protect its new (pounds) 10bn fleet of ''lightweight'' Stryker personnel carriers, which are about to make their active service debut in Iraq, because the vehicles are too thin-skinned to withstand the impact of rocket-propelled grenades.

The Stryker, an eight-wheeled, 19-ton armoured car that weighs about half as much as traditional tracked armoured carriers, was designed to be easily transportable by air to allow American forces to deploy in strength at short notice to global flashpoints.

About 300 of them are about to be sent to Iraq to equip a 3600-man infantry brigade operating in the dangerous ''Sunni triangle'' area of the country north and east of Baghdad.

To enable accelerated development, the vehicle specifications did not feature anti-RPG armour.

It was field tested only against rifle and machine-guns.

Lieutenant Colonel Stephen Barger, a US Army spokesman, confirmed yesterday that the fleet was now being retro-fitted with temporary ''slat'' armour designed to explode RPG warheads prematurely before they can penetrate the vehicles' 13-man crew compartment.

''This will drastically increase their protection. More permanent armour is now being tested,'' he said.

Built by General Dynamics, the Stryker is designed to bridge the gap between light forces such as paratroopers and mechanised units using 53-ton Abrams tanks.