Israeli Prime Minister Benjamin Netanyahu drew the world a stark red line, warning that Iran could have a nuclear bomb in less than a year and demanding international action.
Wielding a red marker pen and a cartoonish diagram of a round bomb with a fizzing fuse, Netanyahu told the UN General Assembly that the international community must put a limit on Tehran's uranium enrichment.
He did not threaten a unilateral attack, but said Iran's uranium enrichment plants would only remain a credible "target" until the middle of next year, when he fears weapons grade fuel will be transferred to smaller bomb labs.
Iran sent an envoy to the assembly to warn that it would "retaliate with full force" against any attack and to demand that the international community "exert pressure on this regime to end all this irresponsible behavior."
The United States has resisted demands to set a precise deadline for action, but foreign ministers from the major powers met after Netanyahu's speech and called for Iran to act "urgently" to answer their nuclear concerns.
"At this late hour, there is only one way to peacefully prevent Iran from getting atomic bombs -- and that's by placing a clear red line on Iran's nuclear weapons program," Netanyahu told the 193-member UN assembly.
"The red line must be drawn on Iran's nuclear enrichment program because these enrichment facilities are the only nuclear installations that we can definitely see and credibly target."
The Israeli leader brought his rough diagram of a bomb -- compared by many observers to the kind of explosives used by the hapless coyote in the Warner Brothers' Roadrunner cartoon -- with him onto the famous UN stage.
Netanyahu claimed Iran is 70 percent of the way toward enriching enough uranium to put itself within reach of a weapon, and used his red marker to indicate the 90 percent line he said was the limit of tolerance.
The Iranian government says it is enriching uranium to 20 percent purity -- a short technical step from the 90 percent needed for a bomb -- for a medical research reactor. But the West believes the effort hides a military goal.
"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," Netanyahu warned.
"From there, it's only a few months, possibly a few weeks, before they get enough enriched uranium for the first bomb."
"Faced with a clear red line, Iran will back down," he added. "Red lines don't lead to war, red lines prevent war."
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Iran's deputy UN ambassador, Eshagh al-Habib, exercised his nation's right of reply and returned to the podium to brand Israel as a "regime which is based on terrorism and is the father founder of state terrorism in the world."
Al-Habib accused Netanyahu of making "baseless allegations" against Iran's "exclusively peaceful" nuclear program.
Netanyahu's denunciation of Iran was one of the fiercest he has made so far. "To understand what the world would be like with a nuclear armed Iran, just imagine the world with a nuclear armed Al-Qaeda," he said.
The speech came after US President Barack Obama said Tuesday that the United States would "do what we must do" to head off an Iranian bomb.
The UN Security Council has passed four rounds of sanctions against Iran while the United States, Russia, China, Britain, France, Germany and the European Union have sought to negotiate with the Islamic state.
Chinese Foreign Minister Yang Jiechi told the General Assembly that, in Beijing's view: "The Iranian nuclear issue has reached a new crucial stage.
But he added: "The relevant parties should remain committed to a diplomatic solution and begin a new round of dialogue as soon as possible."
Foreign ministers from the six-nation contact group met after the speeches to discuss the crisis, and called on Iran to back down and enter talks.
EU foreign policy chief Catherine Ashton told reporters said: "We discussed at length the need for Iran to take action urgently."
Netanyahu and Obama, who have testy relations, are to speak by telephone on Friday. In his address, the Israeli took care to praise Obama: "I very much appreciate the president's position as does everyone in my country."
Eytan Gilboa, an expert on US-Israeli relations at Bar Ilan University in Israel, said Netanyahu had sought to press the United States to name its own "red line" by saying Iran could have a bomb next year.
"This is a bit of a gamble on Netanyahu's behalf," he argued, because Israel will have to attack if Iran's enrichment does not change.
The United States may find the speech extreme, the expert admitted, but it could "spur the US and Europeans to impose tighter sanctions and enter nuclear negotiations with Tehran with tougher stances."