Israel has one year to destroy Iran's nuclear programme or it faces the risk of coming under nuclear attack, the former head of its foreign intelligence agency said in an interview published Sunday.

Speaking to the Sunday Telegraph, Shabtai Shavit said the "worst-case scenario" was that Tehran would have a nuclear weapon within "somewhere around a year".

"The time that is left to be ready is getting shorter all the time," he was quoted as saying by the weekly.

"As an intelligence officer working with the worst-case scenario, I can tell you we should be prepared. We should do whatever necessary on the defensive side, on the offensive side, on the public opinion side for the West, in case sanctions don't work. What's left is a military action."

The chief of Iran's elite Revolutionary Guards General Mohammad Ali Jafari warned Israel not to attack it, saying that the Jewish state was well within range of its missiles, according to a newspaper report Saturday.

Iran has defied UN sanctions and international demands by pressing ahead with its uranium enrichment programme, which both Washington and Israel fear will be used to build a nuclear weapon.

Tehran denies wanting the bomb, and says its nuclear ambitions extend only to generating electricity for a growing population.

Shavit also waded into the American presidential race between Republican John McCain and Democrat Barack Obama, saying that the latter was less likely to approve an Israeli military strike against Iran.

"If McCain gets elected, he could really easily make a decision to go for it," Shavit was quoted as saying.

"If it's Obama: no. My prediction is that he won't go for it, at least not in his first term in the White House."

He warned, however, that American approval was not a necessary pre-requisite for Israel carrying out an air strike on Iranian nuclear facilities.

"When it comes to decisions that have to do with our national security and our own survival, at best we may update the Americans that we are intending or planning or going to do something," he said.

"It's not a precondition, [getting] an American agreement."