Israeli Prime Minister Benjamin Netanyahu drew a 'red line' for Iran's nuclear programme on Thursday despite a US refusal to set an ultimatum, saying Tehran will be on the brink of developing a nuclear weapon in less than a year.

By citing a time frame in an address to the UN General Assembly, Netanyahu - who has clashed with President Barack Obama over the urgency of military action against Iran - appeared to suggest no Israeli attack was imminent before the November 6 US presidential election.

Holding up a cartoon-like drawing of a bomb with a fuse, Netanyahu literally drew a red line just below a label reading 'final stage' to a bomb, in which Iran was 90 per cent along the path to having sufficient weapons-grade material.

Experts put that at the point that Iran has amassed enough uranium, purified to a level of 20 per cent, that could quickly be enriched further and be used to produce an atomic bomb.

Netanyahu told the United Nations he believes that faced with a clear red line, Iran will back down in a crisis that has sent jitters across the region and through financial markets.

"And this will give more time for sanctions and diplomacy to convince Iran to dismantle its nuclear weapons program altogether," said the Israeli leader who later met with US Secretary of State Hillary Clinton for 75 minutes.

Netanyahu's remarks were the closest he or any top Israeli official has come to publicly laying out precisely which Iranian actions could trigger an Israeli military strike on Tehran's nuclear infrastructure.

But by referring to a spring or summer 2013 time frame for Iran to complete the next stage of uranium enrichment, the Israeli leader also seemed to dispel, at least for now, fears that Israel might strike Iran before the US presidential election, 40 days away.

Iran's UN mission responding to Netanyahu's speech accused him of making 'baseless and absurd allegations' and said the Islamic Republic 'reserves its full right to retaliate with full force against any attack'.

Iran called Netanyahu's visual tool "an unfounded and imaginary graph ... used to justify a threat against a founding Member of the United Nations."

Netanyahu's remarks also seemed to deliver a two-part message to the Obama White House - along with Iran's leaders, his most important audience - signaling that the hawkish prime minister wanted an end to the all-too-public war of words with Washington over Iran's suspected nuclear ambitions.

But they also showed he was not backing down from his insistence that harsher warnings must be delivered to Tehran.

A senior State Department official making no mention of Netanyahu's ultimatum said the Israeli leader and Clinton reaffirmed 'that the United States and Israel share the goal of preventing Iran from acquiring a nuclear weapon'.

White House spokesman Jay Carney said he expected Obama to have a follow-up phone call with Netanyahu, probably on Friday.

'NEXT SPRING OR SUMMER'

In his speech, Netanyahu never explicitly said that if Iran crossed his red line, Israel would launch attacks against Iranian nuclear facilities, but he did seem to imply such a threat.

"At this late hour, there is only one way to peacefully prevent Iran from getting atomic bombs. That's by placing a clear red line on Iran's nuclear program," Netanyahu said.

Iran, Netanyahu said, was well into what he defined as the second stage of enrichment - 20 per cent purification - and predicted it would complete that stage by 'next spring, at most by next summer, at current enrichment rates'.

According to an August report by the U.N. International Atomic Energy Agency (IAEA), Iran has stockpiled 91.4 kg (201.5 pounds) of the 20 per cent material.

Some experts say Iran would need 200 to 250 kg (440 to 550 pounds) of such material for a weapon. Other experts suggest less might do it. 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 enrichment centrifuges.

According to the U.N. nuclear watchdog, around 25 kg (55.1 pounds) of uranium enriched to a 90 percent purity level would be needed for a single nuclear weapon.

In his own speech to the General Assembly on Tuesday, Obama said the United States will 'do what we must' to prevent Iran from acquiring nuclear weapons and that time is not unlimited for diplomacy to resolve the issue.

Britain, France, Germany, the United States, Russia and China have negotiated with Iran without success in one form or another for nearly 10 years to persuade it to halt its nuclear program in exchange for political and economic incentives.

Addressing the General Assembly on Thursday, Chinese Foreign Minister Yang Jiechi said disagreement over Iran's nuclear programme had reached 'a new, crucial stage,' and urged a diplomatic solution.

The six nations, whose foreign ministers met at the United Nations on Thursday, have held three rounds of talks with Iran this year without visible progress.

A US official voiced hope for a fourth round 'in the not-too-distant future'.

As if to highlight Netanyahu's concerns that tougher U.N. sanctions against Iran are unlikely due to Russian and Chinese resistance, the group failed to agree on any plan for further steps against Tehran, envoys said.

Obama set no ultimatum or clear 'red line' of his own, despite public urging from Netanyahu over the past several weeks that has aggravated strains between the two leaders.

'CHART A PATH FORWARD'

Seeking re-election, Obama has faced criticism from Republican challenger Mitt Romney that the president is being too tough with Israel and not tough enough with Iran.

"I very much appreciate the president's position, as does everyone in my country. We share the goal of stopping Iran's nuclear weapons program," Netanyahu said.

"Israel is in discussions with the United States over this issue, and I am confident we can chart a path forward together," he said.

He spoke a day after Iranian President Mahmoud Ahmadinejad addressed the General Assembly.

Ahmadinejad said on Monday he did not take seriously the threat that Israel could launch a military strike on Iran's nuclear facilities.

He also said Israel has no roots in the Middle East and would be 'eliminated'.

Obama has drawn criticism from Republicans for opting not to meet Netanyahu or other foreign leaders on the sidelines of the General Assembly and focus instead on his re-election campaign.

Netanyahu has faced opposition within his cabinet and from former Israeli security chiefs to any go-it-alone attack on Iran.

Opinion polls show Israelis are wary of any such strike by their military, whose capability of destroying underground Iranian facilities is limited.

Israel, believed to have the Middle East's only atomic arsenal, sees a nuclear-armed Iran as a threat to its existence and has expressed frustration over the failure of diplomacy and sanctions to rein in Tehran's nuclear activity.

Iran says it is enriching uranium only for peaceful energy and medical purposes, not for nuclear bombs.