Israeli leader Benjamin Netanyahu flourished his marker pen on the floor of the United Nations on Thursday to draw a literal red line across Iran's alleged nuclear ambitions.

The dramatic intervention, in which he used a cartoonish graph based on a diagram of a bomb with a lit fuse to represent Iran's enrichment program, was designed to underscore what he said was an imminent threat.

"The hour is getting late, very late," the prime minister declared, warning that Iran was on course to enrich enough uranium to arm a nuclear device by as early as mid-2013, and demanding that the world intervene to halt it.

"At this late hour, there is only one way to peacefully prevent Iran from getting atomic bombs -- and that is by placing a clear red line on Iran's nuclear weapons program," he declared, deploying his red pen.

"To be credible, a red line must be drawn first and foremost in one vital part of their program -- on Iran's efforts to enrich uranium."

Citing reports from UN nuclear watchdog the International Energy Agency, Netanyahu said that Tehran had already amassed 70 percent of the enriched uranium that it needed despite international economic sanctions.

Netanyahu was apparently referring to a report from the IAEA in August that said Iran had stockpiled 184 kilos of uranium enriched to a 20 percent level, which must be further enriched to 90 percent purity for use in a bomb.

The Islamic republic says it needs to enrich the uranium to the 20 percent level for a medical research reactor, but Israel and much of the West worry it will be put back into the centrifuges and refined to weapons grade.

"By next spring, at most by next summer at current enrichment rates, they will have finished the medium enrichment and moved on to the final stage," the Israeli leader said, referring to the process of creating bomb fuel.

"From there it's only a few months, possibly a few weeks, before they get enough enriched uranium for the first bomb," he claimed. "Nothing could imperil our future more than an Iran armed with nuclear weapons.

He recounted a long list of "terrorist" attacks which he blamed on Iran's Islamic regime, declaring: "Given this record of Iranian aggression without nuclear weapons, just imagine this aggression with nuclear weapons.

"If their terror networks were armed with atomic bombs, who among you would feel safe in the Middle East? Who would be safe in Europe? Who would be safe in America? Who would be safe anywhere?" he asked.

Israel has refused to rule out unilateral action to halt the Iranian nuclear program, but Netanyahu warned that this would be impossible if the fuel was moved out of large-scale enrichment factories and into small weapons labs.

"The red line must be drawn on Iran's nuclear enrichment program because these enrichment facilities are the only nuclear facility installations that we can definitely see and credibly target," he said.

This would imply a window for any Israeli attack in first half of next year, after the US presidential election but before enrichment is complete.

Many of the leaders meeting in New York this week warned against unilateral action. UN Secretary General Ban Ki-moon rejected "shrill talk of war."

But Netanyahu said his plan to impose a clear limit of Iran's actions, one that the international community would enforce in some unspecified way, would head off this danger and force Tehran to abandon its quest.

"Faced with a clear red line Iran will back down," he said. "Red lines don't lead to war, red lines prevent war."

Before the speech, Israeli officials said they were acting in lockstep with the policy of Israel's closest ally, the United States, despite Washington's rejection of the idea of a publicly declared red line for action.

"What he will say regarding red lines will help ensure this goal will be achieved," a Netanyahu aide said. "Netanyahu is convinced that the US and Israel can work together to achieve this common goal."

When he spoke to the UN on Tuesday, Obama certainly had tough words for Iran, warning that the United States would "do what we must" to prevent the Islamic Republic from getting its hands on a bomb.

"Make no mistake. A nuclear-armed Iran is not a challenge that can be contained. It would threaten the elimination of Israel, the security of Gulf nations, and the stability of the global economy," Obama said.

But relations between Netanyahu and Obama are reportedly frosty, and the White House has declared the red line idea unhelpful. Obama and Netanyahu are expected to talk by telephone on Friday, but no meeting is scheduled.

Netanyahu's apocalyptic speech overshadowed one by his Middle East rival Mahmud Abbas, president of the Palestinian Authority, who came to New York to plead to be allowed to join the United Nations with enhanced observer status.

There were also crisis talks between the leaders of the Democratic Republic of Congo and of Rwanda, Joseph Kabila and Paul Kagame, designed to head off new tensions between the neighbors, amid a wave of rapes and killings.

And UN members continued to agonize over the civil war in Syria, with envoys wrangling over ways to halt the bloodshed, and UN-Arab League peace envoy Lakhdar Brahimi lobbying them for strong and united backing for his mission.

burs-dc/ag