Chief of Staff Lt Gen Benny Gantz made the statement in an interview with the Israeli newspaper, Haaretz.
He said Iran's Supreme Leader, Ayatollah Ali Khamenei, had not yet made the final decision whether to build a nuclear bomb.
Tehran says it wants nuclear technology for peaceful purposes but the West believes Iran is developing weapons.
In November, the UN's International Atomic Energy Agency published a report which complained it had been unable to "provide credible assurance about the absence of undeclared nuclear material and activities in Iran" and that it continued to have "serious concerns regarding possible military dimensions to Iran's nuclear programme".
The US, European Union, Canada, Japan and Australia are among those who have imposed sanctions on Tehran.