Correction: An earlier version of the article incorrectly described Haim Saban as Israeli and as the host of the event. The forum was hosted by the Saban Center for Middle East Policy at the Brookings Institution, not by Saban himself, and he is an Israeli American born in Egypt. This version has been corrected.

President Obama speaks at a forum hosted by Haim Saban on Dec. 7 in Washington, D.C. (Pete Marovich/Getty Images)

President Obama on Saturday told an influential audience in Washington that his pursuit of a comprehensive diplomatic deal to end Iran’s development of a nuclear weapon is as likely to fail as succeed, but he defended it as the best option to protect U.S. and Israeli national security with respect to the issue.

“We have to be vigilant about maintaining our security postures, not be naive about the dangers that an Iranian regime poses, fight them wherever they’re engaging in terrorism or actions that are hostile to us or our allies,” Obama said at a forum hosted by the Saban Center for Middle East Policy at the Brookings Institution.

“But,” Obama added, “we have to not constantly assume that it’s not possible for Iran, like any country, to change over time. It may not be likely. If you asked me what is the likelihood that we’re able to arrive at the end state that I was just describing earlier, I wouldn’t say that it’s more than fifty-fifty. But we have to try.”

Obama described the recent deal with Iran — which the administration says halts development of Iran’s nuclear program in exchange for modest relief from sanctions — as a necessary test to see whether a more comprehensive agreement could be secured. He said that while it would be ideal for Iran to pledge to eliminate its entire nuclear capacity, doubts about the regime’s credibility call for a more realistic approach.

“Precisely,” Obama said, “because we don’t trust the nature of the Iranian regime, I think that we have to be more realistic and ask ourselves: What puts us in a strong position to assure ourselves that Iran is not having a nuclear weapon and that we are protected? What is required to accomplish that, and how does that compare to other options that we might take?”

Obama said the international community will have an even greater ability to put new pressure on Iran if Tehran fails to comply with the agreement over the next six months.

“If at the end of six months it turns out that we can’t make a deal, we’re no worse off, and in fact we have greater leverage with the international community to continue to apply sanctions and even strengthen them,” he said. At the same time, he warned, he was not confident the sanctions themselves would have led Iran to abandon its nuclear program.

“[It] is not the choice between this deal and the ideal, but the choice between this deal and other alternatives,” Obama said.

Speaking to the same group later Saturday, Secretary of State John F. Kerry was more optimistic about the chances for a lasting deal with Iran that makes Israel safer.

Kerry tried to sell the deal to the pro-Israel audience as a way to address many of the potential threats from Iran’s nuclear program that Israeli Prime Minister Benjamin Netanyahu has identified as the most pressing. Implicitly addressing criticism from Netanyahu and many pro-Israel lawmakers that the United States is being duped, Kerry stressed that Iran’s compliance will be tested and monitored at every step.

“This is not about trust,” Kerry said.

Lasting peace with the Palestinians also will make Israel safer, and Kerry said his efforts toward a peace deal are focused on Israel’s specific security needs in any final deal to create a Palestinian state.

No Israeli prime minister could sell a peace deal at home if it did not contain carefully considered measures to defend Israel against new threats the deal might open, Kerry said.

Kerry returned Friday night from his eighth trip to Israel in 10 months as the top U.S. diplomat. This visit centered on the presentation of secret proposals for security measures in the West Bank’s Jordan Valley that were developed by retired Marine Gen. John R. Allen, who sat in the audience as Kerry spoke Saturday.

Allen’s work marks the most extensive U.S. study of Israeli security needs in the West Bank, Kerry said. Israel now holds military control of the territory, which would form the bulk of the independent Palestinian state that Kerry envisions. Many Israelis fear that withdrawal would expose Israel to terrorist rockets and other potential threats along the new border.

Anne Gearan contributed to this report.