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President Obama cast doubt on his hard-fought deal with Iran, saying Saturday that he believed  the chances of a final, comprehensive nuclear agreement being struck with Tehran are less than 50-50.

The comments, during  a forum at the Brookings Institution in Washington, came ahead of Israeli Prime Minister Benjamin Netanyahu's expected address to the same forum on Sunday. The Netanyahu government has been sharply critical of the short-term agreement with Iran, and Obama could be trying to smooth over their differences.

However, Obama defended the interim deal struck by the U.S., Iran, and five other world powers last month in Geneva, saying that diplomacy had to be tested as a solution to the crisis and was the best way to prevent Tehran from acquiring automatic weapons.

"If you ask me what is the likelihood that we're able to arrive at the end state ... I wouldn't say that it's more than 50-50," Obama said. "But we have to try."

Obama made the remarks via webcast. The think tank was hosting a forum on U.S.-Israel relations that was broadcast live on Israeli television.

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The president's remark was startling. Obama has tried to allay the fears of many Israelis and some Americans that his administration last month promised to ease economic pressure too much in return for too few Iranian concessions.

The comment nevertheless pointed to the difficult talks that await as the U.S. and its negotiating partners — Britain, China, France, Germany and Russia — work toward a final pact next year. The goal is to eliminate the possibility of Iran assembling a nuclear arsenal, even if any deal might let Iran continue enriching uranium at lower levels not easily convertible into weapons-grade material.

Secretary of State John Kerry, who opened his address to the forum with a discussion of the state of the Israeli-Palestinian peace talks, echoed Obama's skepticism of Iran when he said," As we enter negotiations for a final, comprehensive agreement, we absolutely do so with our eyes wide open, and, as yet, I have to say, unconvinced that Iran will absolutely make all the decisions, the hard decisions necessary to reach such an agreement."

Obama said the six-month interim agreement halts and rolls back central elements of Iran's nuclear program, compelling Tehran to eliminate higher-enriched uranium stockpiles, stop adding new centrifuges and cease work at a heavy water reactor that potentially could produce plutonium. It also provides time to see if the crisis can be averted through negotiation.

"If at the end of six months it turns out we can't make a deal," Obama said, "we are no worse off." U.S. sanctions against Iran will be fully reinstated and even tightened if Iran fails to uphold the agreement, he pledged.

Netanyahu has called the nuclear agreement in Geneva the "deal of the century" for Iran. In an appearance Friday at the same forum, his foreign minister, Avigdor Lieberman, repeated Israel's objections.

Obama acknowledged some "significant tactical disagreements" with Netanyahu, but said U.S. and Israeli bottom-line goals were the same. Netanyahu was on the agenda to speak to the forum Sunday.

Beyond Israel, Sunni Arab countries have expressed concerns about what America's Iran engagement might mean for the balance of power in the region with Shiite-dominate Iran. Saudi Arabian officials even have talked about their own potential nuclear ambitions.

Echoing Obama's effort to reach out to concerned allies, Defense Secretary Chuck Hagel renewed a U.S. push for the sale of missile defense technology and other weapons systems to U.S.-friendly Gulf nations to counter the threat of Iranian ballistic missiles.

In a speech Saturday in Bahrain, Hagel made clear that any final deal on Iran's nuclear program wouldn't end the threat posed by a country the U.S. considers the world's leading state sponsor of terrorism.

On Mideast peace hopes, Obama echoed an optimistic assessment provided by Kerry during a trip to Israel and the Palestinian territories this past week.

The president said his administration had spent much time working with Netanyahu to understand Israel's security needs as part of any two-state solution.

"I think it is possible over the next several months to arrive at a framework that does not address every single detail but gets us to the point where everybody recognizes it's better to move forward than move backward," Obama said.

Still, he said tough decisions await both sides, including the Palestinians' understanding that a transition period will be necessary so no situation arises similar to Hamas' takeover of the Gaza Strip after Israel's 2005 military pullout.

"The Israeli people can't expect a replica of Gaza in the West Bank," Obama said. "That is unacceptable."

The Associated Press contributed to this report