His remarks elaborate on long-held Israeli concerns that Iran is playing for time even as it engages world powers in negotiations aimed at curbing its uranium enrichment drive. Talks are due to resume in Baghdad on May 23.
"They are currently trying to achieve immunity for the nuclear program," Barak told the Israel Hayom newspaper.
"If they arrive at military nuclear capability, at a weapon, or a demonstrated capability, or a threshold status in which they could manufacture a bomb within 60 days - they will achieve a different kind of immunity, regime immunity."
Iran insists that its often secretive uranium enrichment is for peaceful energy and medical needs. At higher levels of purification, such projects can yield fuel for warheads, but Israel and the United States agree Iran has not taken that step.
The International Atomic Energy Agency (IAEA) last year issued a report detailing alleged Iranian research and development activities that were relevant to nuclear weapons, lending independent weight to Western suspicions.
Barak has said Iran is holding off until it can dig in behind defenses sufficient to withstand threatened Israeli or U.S. air strikes on its nuclear facilities.
His 60-day timeline for potential Iranian warhead production appeared aimed at skeptics both at home and abroad of Israel's alarm who say it is too early to rattle sabres.
Israeli leaders believe the diplomatic drive, which involves the five permanent members of the U.N. Security Council, has a low chance of success, and suggest that Iran's rulers seek an atomic bomb as insurance against outside intervention.
Some prominent Israelis have questioned the strategic value of a pre-emptive strike, with former spy chief Yuval Diskin last week accusing the government of promulgating the "false impression" it had the means of halting Iran.
"This is not so. We have been talking all the time about a delay," said Barak, indicating that Israel could not eradicate Iran's nuclear program, but saw value in forestalling it.
Israel is reputed to have the region's only atomic arsenal, but many experts - including U.S. military chief, General Martin Dempsey - have voiced doubt that its conventional forces would be able to deliver lasting damage to Iran's distant, dispersed and fortified facilities.
The idea that some countries with civilian atomic projects might then use them for military purposes is commonplace, letting states keep their options open while not necessarily violating their non-proliferation commitments.
A leaked diplomatic cable from 2008 quoted senior U.S. State Department official John Rood saying Japan was "not a nuclear threshold country...but rather is 'over the threshold' and could develop nuclear weapons quickly if it wanted to" should it feel the need to vie with its nuclear-armed Asian neighbours.
Barak, who leads the sole centrist party in Prime Minister Benjamin Netanyahu's conservative coalition government, has in the past sounded sanguine about Israel's ability to deter a nuclear-armed Iran from attacking.
But with an Israeli election expected in September, and given Iran's nuclear advances as well as Western war jitters, Barak has publicly closed ranks with the hawkish Netanyahu.
In Friday's interview with the pro-government daily, Barak said Iran might regard trying to destroy Israel with nuclear weapons as worth the risk of catastrophic retaliation.
Under such thinking, he said, "after the exchange of strikes, Islam would remain and Israel would no longer be what it was". (Editing by Crispian Balmer and Angus MacSwan)