Should Israel decide to launch a strike on Iran, its pilots would have to fly more than 1,000 miles across unfriendly airspace, refuel in the air en route, fight off Iran's air defenses, attack multiple underground sites simultaneously - and use at least 100 planes.
That is the assessment of American defense officials and military analysts close to the Pentagon, who say that an Israeli attack meant to set back Iran's nuclear program would be a huge and highly complex operation. They describe it as far different from Israel's "surgical" strikes on a nuclear reactor in Syria in 2007 and Iraq's Osirak reactor in 1981.
In a sign of rising American concern, Tom Donilon, the national security adviser, met with prime minister Benjamin Netanyahu of Israel in Jerusalem on Sunday, and the chairman of the joint chiefs of staff, general Martin E Dempsey, warned on CNN that an Israeli strike on Iran right now would be "destabilising." Similarly, the British foreign secretary, William Hague, told the BBC that attacking Iran would not be "the wise thing" for Israel to do "at this moment."
But while an Israeli spokesman in Washington, Lior Weintraub, said the country continued to push for tougher sanctions on Iran, he reiterated that Israel, like the US, "is keeping all options on the table."
The possible outlines of an Israeli attack have become a source of debate in Washington, where some analysts question whether Israel even has the military capacity to carry it off. One fear is that the United States would be sucked into finishing the job - a task that even with America's far larger arsenal of aircraft and munitions could still take many weeks, defence analysts said. Another fear is of Iranian retaliation.