Iran's ruling clerics could use nuclear weapons to strengthen their grip on power and the world must urgently impose crippling sanctions to prevent them from building such arms, Israel's defence minister said on Sunday.

Ehud Barak also predicted that Syria's ruling Assad family could fall within weeks and that this would be a blessing for the Middle East.

Something is wrong with this family, the way they suppress the will of the Syrian people, killing them, slaughtering their own people, he told a conference in Vienna.

Asked about prospects for an Israeli attack on its arch foe Iran's nuclear sites, Barak said he still believed that it was time for urgent, coherent, paralysing punitive steps targeting Iranian oil trade and its central bank.

Nothing short of this kind of sanctions will work, Barak said, adding there was a need for a direct attack, isolation, by the whole world of the Iranian central bank.

Speculation that Israel, which sees Iran's nuclear programme as an existential threat, could launch pre-emptive strikes against Iran was fuelled by a U.N. report last month which said Tehran appeared to have worked on designing a nuclear weapon.

The Islamic Republic, which often lashes out at Israel over its assumed atomic arsenal, says allegations that it is seeking nuclear arms are based on forged evidence.

Barak said he would love to see the Arab Spring jumping over the Gulf into Iran, referring to political upheaval in Egypt, Tunisia, Libya and elsewhere over the last year.

This regime in Iran, the ayatollahs, they will be not be there I believe in 10 or 15 years. It is against the nature of the Iranian people and what happens all around the world.

But if they turn nuclear they might assure another layer of immunity, political immunity for the regime in the same way that Kim Jong-il assured his, Barak said, referring to the North Korean leader and that country's development of nuclear weapons.

He suggested that the Libyan conflict could have taken a different course if Muammar Gaddafi had declared at the outset that he has three or four nuclear devices.

Earlier this month, Barak said that an Israeli attack on Iran was not imminent. He has also said there were several months left in which to decide on such action.

Turning to events in neighbouring Syria, he predicted on Sunday the end of the 41-year rule of the Assad family. They are going to disappear, probably in a few weeks...The falling down of this family is a blessing for the Middle East.

President Bashar al-Assad's forces have cracked down on a revolt that began in March with peaceful protests but has become increasingly violent. Army deserters are now fighting state security forces.

The way the situation develops is beyond our control...we don't think we have to intervene in any way, Barak said.

He said Israel might at some stage have to take more assertive action in Gaza, where Palestinian militants responded with rocket attacks on Israel after an Israeli air strike killed two militants last Thursday.

(Reporting by Fredrik Dahl and Michael Shields, editing by Rosalind Russell)