UPDATE 4-Netanyahu: Iran on brink of nuclear bomb in 6-7 months

* Netanyahu takes to U.S. airwaves to make his case

* Israel sees shorter time frame than ally Washington

* Remarks likely to stoke tensions with Obama

* Iranian commander issues threat against any Israeli attack

By Matt Spetalnick and Dan Williams

WASHINGTON/JERUSALEM, Sept 16 (Reuters) - Israeli Prime Minister Benjamin Netanyahu warned on Sunday that Iran would reach the brink of being able to build a nuclear bomb in just six or seven months, adding urgency to his demand that President Barack Obama set a "red line" for Tehran amid the worst U.S.-Israeli rift in decades.

Taking to the airwaves to make his case directly to the American public, Netanyahu said that by mid-2013 Iran would have 90 percent of the material it needed for an atomic weapon. He again pressed the United States to spell out limits that Tehran must not cross if it is to avoid military action - something Obama has refused to do.

"You have to place that red line before them now, before it's too late," Netanyahu told NBC's "Meet the Press" program, saying that such a move could reduce the chances of having to attack Iran's nuclear sites.

The unusually public dispute between close allies - coupled with Obama's decision not to meet with Netanyahu later this month - has exposed a gaping U.S.-Israeli divide and stepped up pressure on the U.S. leader in the final stretch of a tight presidential election campaign.

It was Netanyahu's most specific explanation yet on why he has become so strident in his push for Washington to confront Tehran with a more forceful ultimatum. At the same time, his approach could stoke further tensions with Obama, with whom he has had a notoriously testy relationship.

U.S. officials say Iran has yet to decide on a nuclear "breakout" - a final rush to assemble components for a bomb - and they express high confidence that it is still at least a year away from the capacity to build one and would then need more time to fit a warhead onto a missile. This contrasts with Netanyahu's timetable, although he stopped short of saying Iran had decided to manufacture a weapon.

Netanyahu showed no signs of backing off from his pressure campaign and equated the danger of a nuclear-armed Iran with the Islamist fury that fueled attacks on U.S. embassies across the Muslim world last week.

"It's the same fanaticism that you see storming your embassies today. You want these fanatics to have nuclear weapons?" Netanyahu asked in the NBC interview, in a clear emotional appeal to Americans still reeling from the angry protests sparked by a film that mocked the Prophet Mohammad.

There have been no accusations, however, of any Iranian role stoking violence that hit Middle Eastern and African capitals.

'IN THE RED ZONE'

Speaking via satellite from Jerusalem, Netanyahu argued that a credible U.S. ultimatum was needed to curb Iran, which denies it is seeking a nuclear bomb.

"They're in the 'red zone,'" Netanyahu said, using an American football metaphor for when a team is close to scoring a touchdown. "You can't let them cross that goal line."

Mohammad Al Jafari, commander-in-chief of Iran's Revolutionary Guards, threatened retaliation for any Israeli attack, saying U.S. bases in the region would be hit and trade via the Strait of Hormuz, a vital oil link, would be disrupted.

"Nothing of Israel would remain," he said.

Susan Rice, Obama's U.N. envoy, offered no sign that Obama - who has asked Netanyahu to hold off on any strike on Iran's nuclear sites to give sanctions and diplomacy time to work - intended to soften his resistance to red lines.

"We will take no option off the table to ensure that (Iran) does not acquire a nuclear weapon, including a military option," Rice told "Meet the Press," reiterating Obama's longstanding position but insisting "they are not there yet."

Israeli leaders, who see Iran's nuclear advances as a looming existential threat, have made clear they are operating on a far tighter window than the United States, which has a superpower's mighty conventional arsenal at its disposal.

Asked whether Israel was closer to acting on its own, Netanyahu said: "We always reserve the right to act. But I think that if we are able to coordinate together a common position, we increase the chances that neither one of us will have to act."

Obama, who is seeking re-election in November, has faced criticism from Republican rival Mitt Romney that the president is being too tough with Israel and not tough enough with Iran.

But Netanyahu took a more neutral posture on the election, denied he was meddling in U.S. politics in support of fellow conservative Romney and distanced himself from the Republican's accusation that Obama was "throwing Israeli under the bus."

Netanyahu's sharpened rhetoric in recent days had fueled speculation that Israel might attack Iran before the U.S. election, believing that Obama would give it military help and not risk alienating pro-Israeli voters.

Netanyahu has drawn criticism at home for overplaying his hand. He faces divisions within the Israeli public and his cabinet that will make it hard to launch a strike any time soon.

He said he appreciated Obama's assurances Iran would not be allowed to obtain a nuclear weapon. But Netanyahu, whose "red line" demands have infuriated U.S. officials, made clear that was not enough. "I think a red line, in this case, works to reduce the chances of the need for military action," he said.

MOVING RAPIDLY

In his most specific comments on Iran's nuclear work, Netanyahu told CNN: "They're moving very rapidly to completing the enrichment of the uranium that they need to produce a nuclear bomb. In six months or so they'll be 90 percent of the way there."

He appeared to be referring to Iran's enrichment of uranium to 20 percent purity, a level it says is required for medical isotopes but which also is close to bomb-fuel grade. According to an August report by U.N. inspectors, Iran has stockpiled 91.4 kg of the 20 percent material.

Experts say about 200-250 kg (440-550 pounds) would be the minimum required to enrich further into enough material for a bomb. Iran could potentially reach that threshold soon by producing roughly 15 kg (33 pounds) a month, a rate that could be speeded up if it activates new uranium centrifuges.

Israel's concern is that Iran be prevented from reaching nuclear weapons capability, not just from developing an actual device, and they worry time is running out. Israel is widely believed to possess the Middle East's only nuclear arsenal.

Netanyahu did not repeat his harshest comments of last week that Washington had lost any "moral right" to restrain Israel because it had refused to put strict U.S. limits on Tehran.

That was followed by word that Obama would not meet Netanyahu during the Israeli leader's U.S. visit later this month to address the United Nations - widely viewed as a snub.