WASHINGTON (AP) _ The United States and Iran took a historic step toward ending more than three decades of estrangement on Friday when President Barack Obama and Iranian President Hassan Rouhani spoke by phone and agreed to work on resolving global suspicions that Tehran is trying to build a nuclear weapon.

   
The 15-minute call capped a week of seismic shifts in the relationship that revolved around Rouhani's participation in the annual U.N. meeting of world leaders. The night before the two leaders spoke, U.S. and European diplomats hailed a ``very significant shift'' in Iran's attitude and tone in the first talks on the nuclear standoff since April.

The diplomatic warming began shortly after Rouhani's election in June. But it is rooted in both presidents' stated campaign desires _ Obama in 2008 and Rouhani this year _ to break through 34-year-old barriers and move toward diplomacy.

Iran is also seeking quick relief from blistering economic sanctions that the U.S. and its Western allies have imposed on Tehran to punish it for refusing to scale back its nuclear activities. Iran insists its nuclear program is for peaceful purposes only, but years of stonewalling inspections and secrecy about its activities have fueled fears it is seeking to build warheads.

Rouhani and Obama spoke while the Iranian president was in his car and headed to the airport to fly back to Tehran, with Obama at his desk in the Oval Office. Rouhani's aides initially reached out to arrange the conversation, and the White House placed the call.

The last direct conversation between the leaders of the two countries was in 1979 before the Iranian Revolution toppled the pro-U.S. shah and brought Islamic militants to power. Obama said the long break ``underscores the deep mistrust between our countries, but it also indicates the prospect of moving beyond that difficult history.''

``While there will surely be important obstacles to moving forward, and success is by no means guaranteed, I believe we can reach a comprehensive solution,'' Obama told reporters at the White House. Iran's nuclear program has been a major concern not only to the United States but to other Middle Eastern nations, especially Israel, and to the world at large.

Earlier, at a news conference in New York, Rouhani linked the U.S. and Iran as ``great nations,'' a remarkable reversal from the anti-American rhetoric of his predecessors, and he expressed hope that at the very least the two governments could stop the escalation of tensions.

"I want it to be the case that this trip will be a first step, and a beginning for better and constructive relations with countries of the world as well as a first step for a better relationship between the two great nations of Iran and the United States of America,'' Rouhani said at the end of his four-day debut on the world stage to attend the annual U.N. General Assembly.

Iran scholar Gary Sick at Columbia University described the events as "breathtaking'' and said the weeks of slow warming led to Friday's dramatic step.

"This is part of a pattern that has led to a real breakthrough,'' Sick said. "And basically what's happening is that the ice that has covered the U.S.-Iran relationship for over the last 30 years is starting to break. And when ice starts to break up, it goes faster than you think.''