WASHINGTON (AP) -- The United States and Iran secretly engaged in a series of high-level, face-to-face talks over the past year, in a high-stakes diplomatic gamble by the Obama administration that paved the way for the historic deal sealed early Sunday in Geneva aimed at slowing Tehran's nuclear program, The Associated Press has learned.

The discussions were kept hidden even from America's closest friends, including its negotiating partners and Israel, until two months ago, and that may explain how the nuclear accord appeared to come together so quickly after years of stalemate and fierce hostility between Iran and the West.

But the secrecy of the talks may also explain some of the tensions between the U.S. and France, which earlier this month balked at a proposed deal, and with Israel, which is furious about the agreement and has angrily denounced the diplomatic outreach to Tehran.

President Barack Obama personally authorized the talks as part of his effort _ promised in his first inaugural address _ to reach out to a country the State Department designates as the world's most active state sponsor of terrorism.

The talks were held in the Middle Eastern nation of Oman and elsewhere with only a tight circle of people in the know, the AP learned. Since March, Deputy Secretary of State William Burns and Jake Sullivan, Vice President Joe Biden's top foreign policy adviser, have met at least five times with Iranian officials.

The last four clandestine meetings, held since Iran's reform-minded President Hassan Rouhani was inaugurated in August, produced much of the agreement later formally hammered out in negotiations in Geneva among the United States, Britain, France, Russia, China, Germany and Iran, said three senior administration officials. All spoke only on condition of anonymity because they were not authorized to discuss by name the highly sensitive diplomatic effort.

The AP was tipped to the first U.S.-Iranian meeting in March shortly after it occurred, but the White House and State Department disputed elements of the account and the AP could not confirm the meeting. The AP learned of further indications of secret diplomacy in the fall and pressed the White House and other officials further. As the Geneva talks appeared to be reaching their conclusion, senior administration officials confirmed to the AP the details of the extensive outreach.

The Geneva deal provides Iran with about $7 billion in relief from international sanctions in exchange for Iranian curbs on uranium enrichment and other nuclear activity. All parties pledged to work toward a final accord next year that would remove remaining suspicions in the West that Tehran is trying to assemble an atomic weapons arsenal.

Iran insists its nuclear interest is only in peaceful energy production and medical research.

The diplomatic gamble with Iran, if the interim agreement holds up and leads to a final pact preventing Iran from acquiring nuclear weapons, could avert years of threats of U.S. or Israeli military intervention. It could also prove a turning point in decades of hostility between Washington and Tehran _ and become a crowning foreign policy achievement of Obama's presidency.

But if the deal collapses, or if Iran covertly races ahead with development of a nuclear weapon, Obama will face the consequences of failure, both at home and abroad. His gamble opens him to criticism that he has left Israel vulnerable to a country bent on its destruction and that he has made a deal with a state sponsor of terrorism.

The U.S. and Iran cut off diplomatic ties in 1979 after the Islamic Revolution and the storming of the U.S. Embassy in Tehran, where 52 Americans were held hostage for more than a year. But Obama has expressed a willingness since becoming president to meet with the Iranians without conditions.

At the president's direction, the United States began a tentative outreach shortly after his inauguration in January 2009. Obama and Iran's supreme leader, Ayatollah Ali Khamenei, exchanged letters, but the engagement yielded no results.

That outreach was hampered by Iran's hardline former president, Mahmoud Ahmedinejad, whose re-election in a disputed vote in June of that year led to a violent crackdown on opposition protesters. The next month, relations seemed at another low when Iran detained three American hikers who had strayed across the Iranian border from Iraq.

Ironically, efforts to win the release of the hikers turned out to be instrumental in making the clandestine diplomacy possible.

Oman's Sultan Qaboos was a key player, facilitating the eventual release of the hikers _ the last two of whom returned to the United States in 2011 _ and then offering himself as a mediator for a U.S.-Iran rapprochement. The secret informal discussions between mid-level officials in Washington and Tehran began.

Officials described those early contacts as exploratory discussions focused on the logistics of setting up higher-level talks. The discussions happened through numerous channels, officials said, including face-to-face talks at undisclosed locations. They included exchanges between then U.S. Ambassador to the United Nations Susan Rice, now Obama's national security adviser, and Iran's envoy to the world body, the officials said. National Security Council aide Puneet Talwar was also involved, the officials said.

The talks took on added weight eight months ago, when Obama dispatched the deputy secretary of state Burns, the top aide Sullivan and five other officials to meet with their Iranian counterparts in the Omani capital of Muscat. Obama dispatched the group shortly after the six powers opened a new round of nuclear talks with Iran in Almaty, Kazakhstan, in late February.

At the time, those main nuclear negotiations were making little progress, and the Iranians had little interest in holding bilateral talks with the United States on the sidelines of the meeting out of fear that the discussions would become public, the U.S. officials said.

So, with the assistance of Sultan Qaboos, officials in both countries began quietly making plans to meet in Oman. Burns, Sullivan and a small team of U.S. technical experts arrived on a military plane in mid-March for the meeting with the Iranians.

The senior administration officials who spoke to the AP would not say who Burns and Sullivan met with but characterized the Iranian attendees as career diplomats, national security aides and experts on the nuclear issue who were likely to remain key players even after the country's elections in the summer.

The goal on the American side, the U.S. officials said, was simply at that point to see if the U.S. and Iran could successfully arrange bilateral talks _ a low bar that underscored the sour state of relations between the two nations.

Beyond nuclear issues, the officials said the U.S. team at the March Oman meeting also raised concerns about Iranian involvement in Syria, Tehran's threats to close the strategically important Strait of Hormuz and the status of Robert Levinson, a missing former FBI agent who the U.S. believes was abducted in Iran, as well as two other Americans detained in the country.

Hoping to keep the channel open, Secretary of State John Kerry then visited Oman in May on a trip ostensibly to push a military deal with the sultanate but secretly focused on maintaining that country's key mediation role, particularly after the Iranian election scheduled for the next month, the officials said.

Rouhani's election in June on a platform of easing sanctions crippling Iran's economy and stated willingness to engage with the West gave a new spark to the U.S. effort, the officials said.

Two secret meetings were organized immediately after Rouhani took office in August, with the specific goal of advancing the stalled nuclear talks with world powers. Another pair of meetings took place in October.

Burns and Sullivan led the U.S. delegation at each of those sessions, and were joined at the final secret meeting by chief U.S. nuclear negotiator Wendy Sherman.

The Iranian delegation was a mix of officials the Americans had met in March in Oman and others who were new to the talks, administration officials said. All of the Iranians were fluent English speakers.

U.S. officials said the meetings happened in multiple locations, but would not confirm the exact spots, saying they did not want to jeopardize their ability to use the same locations in the future. But at least some of the talks are believed to have taken place in Oman.

The private meetings coincided with a public easing of U.S.-Iranian discord. In early August, Obama sent Rouhani a letter congratulating him on his election. The Iranian leader's response was viewed positively by the White House, which quickly laid the groundwork for the additional secret talks. The U.S. officials said they were convinced that the outreach had the blessing of Ayatollah Khameni, but would not elaborate.

As negotiators continued to talk behind the scenes, public speculation swirled over a possible meeting between Obama and Rouhani on the sidelines of the U.N. General Assembly, which both attended in September in New York. Burns and Sullivan sought to arrange face-to-face talks, but the meeting never happened largely due to Iranian concerns, the officials said. Two days later, though, Obama and Rouhani spoke by phone _ the first direct contact between a U.S. and Iranian leader in more than 30 years.

It was only after that Obama-Rouhani phone call that the U.S. began informing allies of the secret talks with Iran, the U.S. officials said.

Obama handled the most sensitive conversation himself, briefing Israeli Prime Minister Benjamin Netanyahu during a Sept. 30 meeting at the White House. He informed Netanyahu only about the two summer meetings, not the March talks, in keeping with the White House's promise only to tell allies about any discussions with Iran that were substantive.

The U.S. officials would not describe Netanyahu's reaction. But the next day, he delivered his General Assembly speech, blasting Rouhani as a ``wolf in sheep's clothing'' and warning the U.S. against mistaking a change in Iran's tone with an actual change in nuclear ambitions. The Israeli leader has subsequently denounced the potential nuclear agreement as the ``deal of the century'' for Iran.

After telling Netanyahu about the secret talks, the United States then briefed the other members of the six-nation negotiating team, the U.S. officials said.

The last secret gatherings between the U.S. and Iran took place shortly after the General Assembly, according to the officials.

There, the deal finally reached by the parties on Sunday began to take its final shape.

At this month's larger formal nuclear negotiations between world powers and Iran in Geneva, Burns and Sullivan showed up as well, but the State Department went to great lengths to conceal their involvement, leaving their names off of the official delegation list.

They were housed at a different hotel than the rest of the team, used back entrances to come and go from meeting venues and were whisked into negotiating sessions from service elevators or unused corridors only after photographers left.