Micah Brose, a privately contracted interrogator working for American forces in Iraq, near the Iranian border, told The Observer that information on Iran is 'gold'. The claim comes after Washington imposed sanctions on Iran last month, citing both its nuclear ambitions and its Revolutionary Guards' alleged support of Shia insurgents in Iraq. Last week the US military freed nine Iranians held in Iraq, including two it had accused of links to the Revolutionary Guards' Qods Force.

Brose, 30, who extracts information from detainees in Iraq, said: 'They push a lot for us to establish a link with Iran. They have pre-categories for us to go through, and by the sheer volume of categories there's clearly a lot more for Iran than there is for other stuff. Of all the recent requests I've had, I'd say 60 to 70 per cent are about Iran.

'It feels a lot like, if you get something and Iran's not involved, it's a let down.' He added: 'I've had people say to me, "They're really pushing the Iran thing. It's like, shit, you know." '

Brose said that reports about Washington's increasingly hawkish stance towards Tehran, including possible military action, chimed with his experience. 'My impression is they're just trying to get every little bit of ammunition possible. If we get something here it fits the overall picture. The engine needs impetus and they're looking for us to find the fuel - a particular type of fuel.

'It now really depends on who gets elected President in the US. If nothing changes in the current course, I'd say military action is inevitable. But we have to hope there will be a change of course.'

He denied ever being asked to fabricate evidence, adding: 'We're not asked to manufacture information, we're asked to find it. But if a detainee wants to tell me what I want to hear so he can get out of jail... you know what I'm saying.'

Other military intelligence officials in Iraq refused to comment, but one said: 'The message is, "Got to find a link with Iran, got to find a link with Iran." It's sickening.'

Last week in Baghdad the US military showed journalists a recently discovered cache of mortars, rocket-propelled grenades and bomb-making materials it claims are of Iranian origin. Rear Admiral Gregory Smith, spokesman for Multi-National Force Iraq, said it was possible they crossed the border before a recent promise by Iran to stop the flow of munitions into Iraq.

He said: 'Iran has had a historic malign influence here in Iraq. They have financed many of the activities of Shia extremist groups. In many cases they have done training, they have actually deployed some of their personnel here in theatre. The Qods Force (Iranian Revolutionary Guards) have come here - we know that, we've got some in detention. They have said in many cases they were not here and intend to support a more peaceful outcome in Iraq and we look for their excellence in achieving that.'

Among the weapons Washington has accused Iran of supplying to Iraqi insurgents are EFPs, or explosively formed projectiles, which fire a slug of molten metal capable of penetrating even the most heavily armoured military vehicle. The number two US commander in Iraq, Lt Gen Ray Odierno, said there has been a sharp decline in the number of EFPs found in Iraq in the last three months.