A government report says that US forces are now using 1.8 billion rounds of small-arms ammunition a year. The total has more than doubled in five years, largely as a result of the wars in Afghanistan and Iraq, as well as changes in military doctrine.

"The Department of Defense's increased requirements for small- and medium-calibre ammunitions have largely been driven by increased weapons training requirements, dictated by the army's transformation to a more self-sustaining and lethal force - which was accelerated after the attacks of 11 September, 2001 - and by the deployment of forces to conduct recent US military actions in Afghanistan and Iraq," said the report by the General Accounting Office (GAO).

Estimating how many bullets US forces have expended for every insurgent killed is not a simple or precisely scientific matter. The former head of US forces in Iraq, General Tommy Franks, famously claimed that his forces "don't do body counts".

But senior officers have recently claimed "great successes" in Iraq, based on counting the bodies of insurgents killed. Maj-Gen Rick Lynch, the top US military spokesman in Iraq, said 1,534 insurgents had been seized or killed in a recent operation in the west of Baghdad. Other estimates from military officials suggest that at least 20,000 insurgents have been killed in President George Bush's "war on terror".

John Pike, director of the Washington military research group GlobalSecurity.org, said that, based on the GAO's figures, US forces had expended around six billion bullets between 2002 and 2005. "How many evil-doers have we sent to their maker using bullets rather than bombs? I don't know," he said.

"If they don't do body counts, how can I? But using these figures it works out at around 300,000 bullets per insurgent. Let's round that down to 250,000 so that we are underestimating."

Pointing out that officials say many of these bullets have been used for training purposes, he said: "What are you training for? To kill insurgents."

Kathy Kelly, a spokeswoman for the peace group Voices in the Wilderness, said Mr Bush believed security for the American people could come only from the use of force. Truer security would be achieved if the US developed fairer relations with other countries and was not involved in the occupation of Iraq. The President, said Ms Kelly, should learn from Israel's experience of "occupying the Palestinians" rather than buying its ammunition.

The GAO report notes that the three government-owned, contractor-operated plants that produce small- and medium-calibre ammunition were built in 1941.

Though millions of dollars have been spent on upgrading the facilities, they remain unable to meet current munitions needs in their current state. "The government-owned plant producing small-calibre ammunition cannot meet the increased requirements, even with modernisation efforts," said the report.

"Also, commercial producers within the national technology and industrial base have not had the capacity to meet these requirements. As a result, the Department of Defense had to rely at least in part on foreign commercial producers to meet its small-calibre ammunition needs."

A report in Manufacturing & Technology News said that the Pentagon eventually found two producers capable of meeting its requirements. One of these was the US firm Olin-Winchester.

The other was Israel Military Industries, an Israeli ammunition manufacturer linked to the Israeli government, which produces the bulk of weapons and ordnance for the Israeli Defence Force.

The Pentagon reportedly bought 313 million rounds of 5.56mm, 7.62mm and 50-calibre ammunition last year and paid $10m (about £5.5m) more than it would have cost for it to produce the ammunition at its own facilities.