Iraqi arms dumps overrun by coalition forces in 2003 were still being looted by insurgents in search of explosives and ammunition as late as last October, according to a damning new report by the US Government Accountability Office (GAO).

Thousands of tons of military-grade high explosives as well as shells capable of being converted into the kind of roadside booby-traps which caused the majority of US casualties have been taken without hindrance, the report states.

The watchdog GAO also claims that a shortage of military manpower to guard Saddam-era weapons depots meant that insurgents were left free to strip more than 400 sites of lethal hardware for a guerrilla campaign which has killed more than 3000 American and 134 British soldiers.

Although allied troops have disposed of 417,000 tonnes of munitions in controlled explosions since the invasion, Pentagon officials admitted yesterday that they had no hard figures for how much was there to begin with. The GAO agrees the extent of Saddam's arsenal remained "an unknown quantity", but added it might range from thousands to millions of tonnes of conventional munitions.

The GAO report says that, despite these efforts, "unguarded sites will likely continue to support terrorist attacks throughout the region."

According to the GAO, a major factor in the failure to secure arms and ammunition was the assumption post-conflict resistance was unlikely" and that Saddam's army would capitulate and continue to provide security.

Instead, the White House ordered the 400,000-strong army disbanded and the Pentagon policy of committing as few US ground troops as possible backfired.

A Pentagon spokesman said: "We are doing our best to locate and destroy stockpiles, but given the size of the country and the other tasks coalition forces are undertaking, it's a huge task."