The magazine report reveals that congressional leaders agreed to a request from President George Bush late last year for $400m (£200m) for measures described in a "presidential finding", a highly classified document which must be issued when a covert intelligence operation gets under way.

However, Democratic leaders appear to be troubled by the escalation of the cross-border activities inside Iran, particularly the authorisation of lethal force by US special forces as they pursue "high value targets" , which do not appear to be covered by the finding, the investigative journalist Seymour Hersh reports.

The finding focused on undermining Iran's nuclear programme "and trying to undermine the government through regime change," by working with opposition groups inside Iran and by "passing money".

Clandestine activities by the US against Iran are not new, but the scale and the scope of the operations, involving the CIA and the Joint Special Operations Command, have now been expanded, according to current and former officials quoted by Hersh.

"Senior Democrats in Congress told me that they had concerns about the possibility that their understanding of what the new operations entail differs from the White House's," the article says.

At a time when Israeli officials and experts have been raising the prospect of a military strike against Iran before it fully masters the technology of enriching uranium to weapons grade, The New Yorker also highlighted resistance by the US military to bombing Iran. "The Joint Chiefs of Staff, whose chairman is Admiral Mike Mullen, were 'pushing back very hard' against White House pressure to undertake a military strike against Iran," the article says.