WASHINGTON — Ending the dispute over Iran’s nuclear program could ease the hostile relationship between Tehran and the West, resolve one of the world’s most urgent security threats and provide a diplomatic triumph for President Barack Obama.

But as negotiators from Iran and six global powers gather in Vienna today to begin an intense final week of talks before a Nov. 24 deadline, those prizes appear far off, if not out of reach.

“We will have gaps to close,” a senior administration official said Monday, speaking anonymously under State Department ground rules. “We still do not know if we will be able to do so.”

Obama has put the chances of success at 50 percent or less. Many officials of the seven countries say the most likely outcome is a deal to continue the talks, maintaining the current uneasy status quo that keeps Iran’s nuclear ambitions in check — but continues to pose a potential danger.

Yet a chance remains that negotiators will reach an accord, if only because failure would leave problems they desperately need to resolve.

Iran’s oil-based economy would remain isolated from much of the world and face additional pressure from a global slump in oil prices. The West would risk Iran enriching more uranium and edging closer to bomb-making capability, raising the risk of war.

“There still may be a midnight surprise,” said a Western official, suggesting the group could announce a comprehensive deal around midnight on the 24th, just as its members announced an interim deal to continue negotiations in the final hours of talks last year. He declined to be identified, citing diplomatic sensitivities.

The United States, Britain, China, France, Germany and Russia have sought to craft a deal that would gradually lift harsh sanctions imposed on Iran by the U.S., Europe and the United Nations Security Council, in exchange for intrusive inspections and other restrictions to prevent Tehran from gaining enough enriched uranium and the know-how to build a bomb.

Iran insists it is interested only in enriching uranium for nuclear power and other peaceful uses, not weapons.

The U.S. and other negotiators seek to extend the time Iran would need to enrich enough uranium for a single bomb, should its leaders choose that path, from a current estimated “breakout” period of six months to a year or more. That, the thinking goes, would give the outside world enough time to recognize the threat and intervene.

Among the ways to stretch that timeline would be to disable or destroy many of Iran’s 19,000 centrifuges, which refine uranium to make it suitable for bomb fuel, or to reduce its stockpile of already-enriched uranium.