Israel's outgoing home front defence minister says an attack on Iran would likely trigger a month-long conflict that would leave 500 Israelis dead.
Matan Vilnai told the Maariv newspaper that the fighting would be "on several fronts", with hundreds of missiles fired at Israeli towns and cities.
Israel was prepared, he said, though strikes on Iran's nuclear facilities had to be co-ordinated with the US.
Meanwhile, a US blogger has published what he says are Israel's attack plans.
Richard Silverstein told the BBC he had been given an internal briefing memo for Israel's eight-member security cabinet, which outlined what the Israeli military would do to prevent Iran developing nuclear weapons.
Tehran insists its nuclear programme is entirely peaceful.
The purported leaked Israeli memo suggests that the military operation would begin with a massive cyber-attack against Iran's infrastructure, followed by a barrage of ballistic missiles launched at its nuclear facilities.
Military command-and-control systems, research and development facilities, and the homes of senior figures in nuclear and missile development would also be targeted.
Only then would manned aircraft be sent in to attack "a short-list of those targets which require further assault".
BBC diplomatic correspondent Jonathan Marcus says it is not possible to verify the authenticity of the document, but the proposed mission would be huge and have potentially far-reaching consequences.
Iran's government and military have made it clear that if it is attacked either by Israel or the US, it will respond in kind, either directly or through proxies.
In his interview with Maariv, Mr Vilnai said Israel had "prepared as never before".
"There is no room for hysteria," said the former general, who is stepping down at the end of August to become Israel's ambassador to China.
He echoed an assessment by Defence Minister Ehud Barak, who said that it was believed that some 500 people in Israel might be killed.
"There might be fewer dead, or more, perhaps... but this is the scenario for which we are preparing, in accordance with the best expert advice."
"The assessments are for a war that will last 30 days on several fronts," he added, alluding to the possibility of attacks by the Iranian-backed Lebanese Shia Islamist movement, Hezbollah, and Palestinian Islamist militants in the Gaza Strip.
Mr Vilnai also declined to comment on US Defence Secretary Leon Panetta's assertion on Tuesday that Washington did not believe Israel had yet made a decision on whether or not to launch a strike on Iran.
"I don't want to be dragged into the debate," he added. "But the United States is our greatest friend and we will always have to co-ordinate such moves with it."
On Tuesday, Israeli Prime Minister Benjamin Netanyahu announced that Mr Vilnai would be succeeded by Avi Dichter, a former head of Israel's internal security agency, Shin Bet.