Israeli Prime Minister Benjamin Netanyahu has demanded the international community impose what he calls a red line on Iran to prevent it from enriching enough uranium for a nuclear bomb.
Mr Netanyahu told the United Nations General Assembly that Iran is already 70% of the way through the process of enriching enough uranium to fuel a bomb, and must be prevented from reaching the 90% level, AFP reports.
In his address on Thursday, the Israeli leader used a cardboard diagram of a bomb with different levels on it, drawing a thick red line across it with a marker pen for dramatic effect.
He said faced with a clear red line, Iran would back down and imposing an ultimatum on Tehran would not provoke war but help prevent it.
Iran denies it is building a nuclear weapon and has dismissed the Israeli threat.
United States President Barack Obama vowed in his address to the UN on Tuesday that he would prevent Iran from getting the bomb but his administration has repeatedly rejected imposing a red line on Tehran.
"At stake is the future of the world," Mr Netanyahu said. "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, and warned that "given this record of Iranian aggression without nuclear weapons, just imagine this aggression with nuclear weapons.
"If their terror networks 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.
Mr Netanyahu warned that the international community could not rely on its own arsenals to deter Iranian aggression, denouncing Iranian leaders as religious fanatics who would be quite prepared to sacrifice their own population.