The US is preparing to take staff away from the Iraq Survey Group, the CIA-led unit charged with searching for weapons of mass destruction, to join the search for much more basic arms, such as mortars, rocket-propelled grenades, heavy machine guns, and small surface-to-air missiles, which pose an immediate threat to US forces.

American troops have recently been subjected to more audacious, carefully planned attacks, with the guerrillas showing "resilience and creativity", according to the International Institute for Strategic Studies.

"They have abducted and murdered soldiers, shot individual soldiers, melted into crowds and used grenades, mortars, landmines and im provised explosive devices incorporating military munitions", it says in its latest Strategic Comments bulletin.

Many such weapons, it says, exist in some 1,000 unguarded depots around Iraq. The US and other occupying forces have found 5,000 depots which are now under their control. But there remain tens of thousands of small arms and hundreds of surface-to-air missiles either hidden or in the hands of opposition groups.

Saddam Hussein had some 400,000 men under arms and 850 surface-to-air missile launchers, the IISS says.

There is no reliable estimate of the number of weapons still unaccounted for. "We just don't know," an official told the New York Times last month.

"The weapons are everywhere," Ray Lopez, an FBI bomb expert, told the Houston Chronicle. "There are so many sites for ammunition, some we still haven't even found."

Paul Bremer, the chief administrator in Iraq, said on Saturday that coalition forces had found 650,000 tonnes of ammunition since the end of major hostilities. "We think there are another 350,000 tonnes out there," he said.

Lieutenant General Ricardo Sanchez, the commander of US ground forces in Iraq, said: "Every day we find caches, huge caches. We're conducting security operations at the major sites to secure them and prevent ordnance falling into the wrong hands."

The US is offering a reward of $500 (£295) for each shoulder-fired heat-seeking surface- to-air missile handed in. More than 300 have been collected since May 1, according to the American military. It has paid $100,000 in rewards.

But shoulder-launched missiles can fetch much more - perhaps as much as $5,000 - when sold clandestinely. Missiles such as the SA-7 are easily smuggled, weighing less than 14kg and measuring less than two metres in length.

Lt Gen Sanchez said in September that the threat of SA-7 missiles was "a key issue". But heavier weapons are now being used. The Baghdad hotel where Paul Wolfowitz, the US deputy defence secretary, was staying last month was hit by rockets from a multiple launcher powered by a car battery.

The lack of accounting for such missiles is the main reason why the occupation authorities have not yet reopened Baghdad International Airport to commercial traffic, officials said. The terminal has been rebuilt and the runways repaired, and Australian soldiers are running the air traffic control system.

A coalition military spokesman said last night that in the last seven months there had been reports of more than 30 missiles being fired at aircraft as they approached the airport. Planes carrying aid workers and other civilians now practise a stomach-churning "corkscrew" manoeuvre as they start their descent. "I'm told it's the best way to avoid being hit," the spokesman said.

After the Iraqi army collapsed, one of the country's main nuclear facilities went unguarded for at least two weeks.

The Tuwaitha research facility contained nearly two tonnes of partially enriched uranium, and 94 tonnes of natural uranium, which could be processed to form the core of a nuclear weapon.

Tuwaitha was one of seven facilities looted before it could be secured. At least 8kg of uranium are missing.