Dan Shapiro's message resonated far beyond the closed forum in which it was made: Iran should not test Washington's resolve to act on its promise to strike if diplomacy and sanctions fail to pressure Tehran to abandon its disputed nuclear program.

Mr Shapiro told the Israel Bar Association the US hopes it will not have to resort to military force.

"But that doesn't mean that option is not fully available. Not just available, but it's ready," he said. "The necessary planning has been done to ensure that it's ready."

Iran says its nuclear program is for peaceful purposes, like energy production. The US and Israel suspect Iran is pursuing nuclear weapons, but differences have emerged in how to persuade Tehran to curb its program.

Washington says diplomacy and economic sanctions must be given a chance to run its course, and is taking the lead in the ongoing talks between six global powers and Iran.

Israel, while saying it would prefer a diplomatic solution, has expressed scepticism about these talks and says time is running out for military action to be effective.

President Barack Obama has assured Israel that the US is prepared to take military action if necessary, and it is standard procedure for armies to draw up plans for a broad range of possible scenarios. But Mr Shapiro's comments were the most explicit sign yet that preparations have been stepped up.

The five permanent members of the UN Security Council and Germany are gearing up to for a May 23 meeting with Iran in Baghdad. Shortly after the meeting, the UN atomic agency is to release its latest report card on Iran's nuclear efforts.

In Tehran, top nuclear negotiator Saeed Jalili warned against Western pressure at next week's talks, which are a follow-up to negotiations in Istanbul last month that all sides praised as positive.

"Cooperation is what we can talk about in Baghdad," Jalili said in comments broadcast on Iranian state TV.

"Some say time is running out for the talks," he added. "I say time for the (West's) pressure strategy is running out."