Dennis Ross at the Begin Center

If diplomacy fails to stop Iran's nuclear program, the U.S. will use military force, Dennis Ross, a former top adviser to U.S. President Barack Obama, said on Wednesday.

Speaking at a symposium at the Menachem Begin Heritage Center in Jerusalem, Ross said that people should believe Obama's declarations that he will not allow Iran to obtain nuclear weapons.

"When the administration began and [Obama] made it very clear that he was prepared to pursue engagement with the Iranians, it was always a means, it was never an end. It was a means to see if you could affect and change Iran's behavior on the nuclear issue by dealing with them, but it was a means to try to do that. It was also a means recognizing that this was a way to build pressure on Iran," Ross said.

"If we reached out and the Iranians weren't responsive it would be far easier to mobilize the world to put real pressure on the Iranians. But you had to ask the question; what happens if we don't succeed?," he said. "If we do everything we can but it doesn't succeed, are we prepared to live with an Iran that has a capability and then contain it afterwards, or do we believe that the acquisition of that capability is so profoundly threatening to our interests that we really can't live with it and we have to prevent it?"

"If diplomacy fails, and I'm asked to give my advice, it is very important that having stated prevention as an objective that we act on it," he added.

"When President Obama says 'I don't bluff', I think he means what he says. If diplomacy doesn't work, we have to be prepared to use force, and I think we will be."

In the 1990s, Ross was appointed President Bill Clinton's Middle East envoy and was involved in brokering peace talks between Israel and the Palestinians. More recently, Ross served as an adviser to Obama on Middle East affairs, before leaving the post in 2011.

At the symposium Wednesday, Ross argued that history had showed that second-term presidents act much as they did in their first terms, suggesting that Obama will not adopt a tougher stance toward Israel over the next four years, contrary to the pessimistic assessments of some Israeli media commentators.

Ross said that U.S.-Israel security cooperation, which was already good under previous presidential administrations, got even better during Obama's first term.

Ross also said the U.S. would remain consistent in its stance that Iran could not be permitted to get nuclear weapons, even with personnel changes at places like the Pentagon, where Chuck Hagel is expected to replace Leon Panetta as defense secretary in the near future.