"It would be a disaster for the United States and for everyone else," Netanyahu said as a deadline loomed for an agreement on Iran's disputed nuclear program between the West and the Islamic Republic.
Netanyahu's comments in an interview with Fox News Sunday came as foreign ministers of the United States, France, Britain and Germany gathered in Vienna to face off with Iranian counterpart Mohammad Javad Zarif.
A six month interim deal with Iran expires July 20 and US Secretary of State John Kerry said "significant gaps" remain before a final deal can be reached.
"I certainly hope that doesn't happen. I think it would be a catastrophic development, because you know the Middle East is in turmoil, everything is topsy-turvy, the worst militants, Shiites and Sunni radicals are vying with each other who will be the king of the Islamist hill," he said.
"If any one of these sides get their hands on nuclear weapons, all bets are off.