Diplomatic relations between Britain and the United States over Iran are under increasing strain after Gordon Brown's special security adviser warned that American claims about Tehran's military capability should be taken 'with a pinch of salt'.

As a new conservative campaign group with links to the White House prepares to make the case that Iran is a direct threat to the US, Patrick Mercer urged scepticism towards any US justification for strikes against the country.

Mercer, the former shadow homeland security spokesman, who visIted the Iranian capital recently, said: 'There is increasing concern about the apparent evidence that America is preparing about Iranian military involvement.'

Mercer, who last month accepted a post as an adviser to the Brown government, said: 'All that I heard when I was in Iran was British authorities saying "be careful about what you hear from America". I'm not saying for one moment that it is necessarily wrong, but it's got to be taken with a pinch of salt. Is it American rhetoric, propaganda or fact?'

However he conceded that British military commanders had discussed the issue of Iran with their US counterparts, although there had been no offer of support for a US-led air strike against the country. 'Iran is a problem, there's no doubt about it. Whatever is going to occur vis à vis Iran is going to include at the very least British diplomatic effort.'

Meanwhile, a well-funded new conservative campaign group with links to the White House is making the case that Iran is a threat to the US and Israel and should be 'stopped'. Freedom's Watch was founded in March by a dozen billionaire and multimillionaire benefactors. Critics say the organisation is a neo-conservative 'slush fund' and front for White House policy, in particular the views of Vice President Dick Cheney.

The group has paid $15m for media ads supporting the Bush strategy in Iraq at the time that Congress was deliberating the report of General David Petraeus. In the ads it directly linked the 11 September terrorist attacks to the war in Iraq despite the fact that the 9/11 Commission report refuted any such links.

More recently Freedom's Watch took out ads describing Iran's President Mahmoud Ahmadinejad as 'a terrorist'. According to the New York Times, anonymous benefactors told the paper that the group plans to sponsor a private forum of experts on Islam to make the case that Iran poses a direct threat to the US.

The group has close links with the White House: its president, Bradley Blakeman, is a former deputy assistant to Bush. Among its founders is Mel Sembler, a Florida shopping centre magnate who helped to finance the 2000 Florida recount campaign that gave Bush his first presidency. Another is former White House press secretary Ari Fleischer.

But it is the link with Cheney that is the strongest, leading to the widespread view that the organisation is in effect his own creature. The group was conceived in March following a meeting of the Republican Jewish Coalition at which he was a keynote speaker. Last month it emerged that Cheney had asked a number of right-wing think tanks to start building the case for war against Iran.