A strike on Iran would be 'insane,' Mohamed ElBaradei, chief of the International Atomic Energy Agency (IAEA), said in a magazine interview released on Saturday.
Tehran should engage with the United States and negotiate over its nuclear program, he said.
"It would be completely insane to attack Iran," ElBaradei said. "That would turn the region into one big fireball, and the Iranians would immediately start building the bomb - and they could count on the support of the entire Islamic world."
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US President Barack Obama is actively seeking to engage Iran on a series of issues, from its nuclear program to Afghanistan.
"I advise my Iranian negotiating partners: grasp the hand that Obama is extending to you," ElBaradei told Germany's Der Spiegel magazine.
Asked what he meant exactly, the head of the UN nuclear watchdog added: "I think Freeze for Freeze is the next realistic step. The Iranians would install no more centrifuges, the West would forego further sanction measures. During this time, there would be intensive negotiations."
The United Nations Security Council has imposed three rounds of sanctions on Iran for refusing to halt enrichment. Obama's administration has made clear that any overtures to Iran will be accompanied by ramped up sanctions if there is no cooperation.