Biden, speaking at a forum on the Middle East at the Brookings Institution think tank, rejected calls for more sanctions against Tehran over its nuclear program because "this is not the time to risk a breakdown when we still have a chance for a breakthrough. "

Biden said the sanctions imposed by the United States and allies were working by slowing Iran's economy, as well as its nuclear program.

"It's frozen the program, it's given us a shot for a peaceful solution," he said. "I tell you, I think it's a less than even shot but it's a shot, nonetheless."

Biden said that while the United States and Israel have tactical disagreements, Washington remained fully dedicated to Israel's security.

"We will not let Iran acquire a nuclear weapon - period," he said. "End of discussion. Not on our watch."

Last month Iran said it would not make excessive concessions on its nuclear program, which Tehran says is for energy-creation purposes, as part of a 12-year dispute.