Britain's ambassador to Iran gave the US a private masterclass on how to negotiate with Iran, warning of a cumbersome decision-making process and the need to persuade a deeply suspicious republic that time was not on its side in its confrontation with the west.

Geoffrey Adams distilled the lessons from his own experience as the UK representative in Tehran, especially in dealing with the crisis in which 15 British navy personnel were captured by Revolutionary Guards in March 2007.

Now political director at the Foreign Office, Adams briefed the US ambassador to Iraq, Ryan Crocker, and the commander of US and coalition forces in Iraq, General David Petraeus, in Baghdad in November 2007. The US had not had an embassy in Iran since the 1979 Islamic revolution and was preparing for talks with Tehran about the security situation in Iraq.

Adams recommended "being steady and firm, tough but not aggressive, and at the same time, seeking to engage", according to the US state department cable reporting the session.

"Iranians are obsessed with the west and this obsession at times blinds them to their interests," he told the Americans. "The US side should be aware of the following preconceptions on the Iranian side:

– The US seeks to remove the current regime and replace it with a pro-western one.

– US policymakers spend an inordinate amount of time and energy thinking about (and plotting against) Iran. As such, Iranians assume that everything we do or say has meaning and has been carefully thought out and co-ordinated, both internally and with the UK; there are no accidents."

Iran did not believe the threat of US military action against them was realistic, Adams said. It saw the US as a "tough, determined adversary that can be manipulated and wounded".

He warned the Americans to keep in mind the legacy of the Iran-Iraq war, from 1980-1988, during which the international community was either silent or sided with Saddam Hussein (even when he used weapons of mass destruction against Iranians).

Iranian officials arrived at negotiations "with precise and detailed instructions from which they do not deviate, out of fear of professional repercussions", he said. The US should not expect substantive responses to questions or initiatives in real time. Although this problem could be obviated to an extent by scheduling lots of breaks to allow Iranian officials time to confer and seek guidance from Tehran.

"Iranians are not stupid," Adams said repeatedly. "They follow all the issues closely, even if they at times misread the signals."

The short leash on Iranian negotiators, plus the "collaborative and multi-polar nature" of their decision-making and the extreme sensitivity of direct US-Iranian talks, meant the US should not expect immediate action, he said.

"Any progress will be slow and come only after much deliberation and discussion in Iran, with push-back from those circles opposing talks. If the Iranian government concludes that a policy shift is in their interests, it can do it and find a way to justify it, but will first have to achieve internal consensus.

"It is important with the Iranians not to lose one's temper or show that one is upset. Trading accusations allows the Iranian government to rely on a familiar script; far better is to ask unexpected questions that will take them off-script." Adams recommended keeping the tone matter-of-fact and raising issues in question form. He advised against offering an agenda beforehand, "otherwise the agenda itself will become subject to ongoing negotiations".

Iranian interlocutors "assume that time is on their side and hence default towards inaction. Convince them that now is the time to act, and that not doing so will hurt their interests and that the situation could get worse."

He told the Americans to ask for clarification of actions that contradicted their stated commitments.

"Keep your message clear, including redlines, and do not be too subtle."