It is using hitmen, sabotage, front companies and double agents to disrupt the regime's illicit weapons project, the experts say.

The most dramatic element of the "decapitation" programme is the planned assassination of top figures involved in Iran's atomic operations.

They privately acknowledge the new US administration is unlikely to sanction an air attack on Iran's nuclear installations and Mr Obama's offer to extend a hand of peace to Tehran puts any direct military action beyond reach for now.

The aim is to slow down or interrupt Iran's research programme, without the gamble of a direct confrontation that could lead to a wider war.

"The goal is delay, delay, delay until you can come up with some other solution or approach. We certainly don't want the current Iranian government to have those weapons. It's a good policy, short of taking them out militarily, which probably carries unacceptable risks."