WASHINGTON - President Obama said Saturday he believed the chances for a comprehensive nuclear agreement with Iran were 50-50 or worse, yet he defended diplomacy as the best way to prevent Tehran from acquiring atomic weapons.

During a question-and-answer session with a pro-Israel audience, Obama said he wasn't naive about the odds of success for a final agreement between world powers and Iran next year, building on the recent six-month interim deal.

"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."

The president's remark was somewhat startling. He 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.

His comment nevertheless underscored the difficult talks that await as the United States 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's assembling a nuclear arsenal, even if any deal might let Iran continue enriching uranium at lower levels not easily convertible into weapons-grade material.

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

"What we do have to test is the possibility we can resolve this issue diplomatically," Obama said.

"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.

Obama's appearance at the Brookings Institution forum appeared directed as much at an Israeli audience as an American one. The discussion was broadcast live on Israeli television, with analysts there viewing it as an effort to patch over Obama's public differences with Israeli Prime Minister Benjamin Netanyahu.

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.