James Clapper, US director of national intelligence, also says Tehran is keeping option open to develop nuclear weapons

America's top intelligence official has claimed that Iran's leadership was now more willing than before to carry out an attack inside the US, and that intelligence agencies were worried about plots against US and allied interests around the world.

In a report to Congress, the director of national intelligence, James Clapper, said a plot to blow up the Saudi ambassador in a Washington restaurant - which the US blamed on Iran's Revolutionary Guard - "shows that some Iranian officials, probably including supreme leader Ali Khamenei, have changed their calculus and are now more willing to conduct an attack in the United States in response to real or perceived US actions that threaten the regime".

Clapper added: "Iran's willingness to sponsor future attacks in the United States or against our interests abroad probably will be shaped by Tehran's evaluation of the costs it bears for the plot against the ambassador as well as Iranian leaders' perceptions of US threats against the regime."

US officials have expressed concern that the Revolutionary Guards' Quds Force, responsible for external operations, could have sleeper cells in the US or over the border in Mexico and the rest of Latin America.

Clapper's statement was the latest salvo in a war of words between the west and Iran over Tehran's nuclear programme and planned oil embargos by both the US and European Union.

Clapper said Iran was "keeping open the option to develop nuclear weapons".

"We do not know, however, if Iran will eventually decide to build nuclear weapons," Clapper said, pointing out in answer to questions from senators that there were "certain things [the Iranians] have not done" that would be necessary to build a warhead.

The intelligence chief said despite the threatened oil embargos, due to take effect in five months' time, "Iran's economic difficulties probably will not jeopardise the regime".