U.S. President Barack Obama says he has spoken by phone with Iranian President Hassan Rouhani and both men agreed to work "expeditiously" toward an agreement on Iran's nuclear program.

The conversation was the first between the U.S. and Iranian presidents in over 30 years.

Obama says after speaking with Rouhani, he believes the U.S. and Iran can reach a comprehensive solution over Iran's nuclear program.

In a televised address from Washington on Friday afternoon, Obama said the two leaders have directed teams to work “expeditiously” to reach an agreement on Iran's nuclear future.

He stressed that a path to “meaningful agreement” would be difficult, but that there is a unique opportunity to make progress with the new government of Iran.

“The test will be meaningful, verifiable and transparent actions which could lead to lessening of sanctions,” Obama said, underscoring a deep history of mistrust between the two countries.

Iran's President Hassan Rouhani

Iran's President Hassan Rouhani takes questions from journalists during a news conference in New York on Friday after he spoke on the phone with U.S. President Barack Obama. (Adrees Latif/Reuters)

He said the U.S. will co-ordinate closely with its allies, including Israel, which considers an Iranian nuclear weapon capability to be an existential threat.

Iranian and UN officials have been meeting to continue talks on how to investigate suspicions that Iran has worked secretly on trying to develop nuclear weapons. Tehran denies that claim.

A series of tweets sent from Rouhani's account thanked president Obama for the phone call. He also tweeted, in regards to the "nuclear issue, with political will, there is a way to rapidly solve the matter."

Rouhani's new Iran?

Speaking at a news conference in New York, where he attended this week's opening of the UN General Assembly's new session, Rouhani said that after meetings between Iranian officials and the so-called P5+1 countries — Germany, China, Russia, the United States, Britain and France — the outlook is positive for some kind of deal.

"We hope that these talks would, in a short time, yield tangible results," Rouhani said through a translator.

On Friday, Obama acknowledged a new tone in relations between the U.S. and Iran, hinting at a rapprochement between Tehran and states that have shunned it for years. He also mentioned the possible lifting of some of the sanctions Iran has faced.

It's a change of tone Rouhani noted as well.

"In speaking with senior European officials, and also hearing Mr. Obama, the president of the United States, it seemed that they sounded different than compared with the past, and I view that as a positive step in the settlement of the differences between the Islamic Republic of Iran and the West," he said.

Rouhani — who has come across as a more moderate face to the hardline clerical regime in Tehran — said his election in June, before coming to power in August, has created a "new environment" within Iran but also globally, that is serving to "pave the way for better relations."

The news conference capped off a week-long diplomatic push by the new Iranian administration to court Western countries, a marked departure from the standoffish stances taken by previous president Mahmoud Ahmadinejad.

His pronouncements at the UN have raised guarded hopes that progress might be possible. But they have also served as a reminder that the path to that progress will not be quick or easy.

During discussions in Vienna, home of the International Atomic Energy Agency, Iranian envoy Reza Najafi stressed the new government's policy of "constructive interaction" but also said he did not expect any agreement in the immediate future.

"This is the first meeting, so nobody, I guess, should expect that in just a one-day meeting we can solve [our] problems," Najafi, appointed as ambassador last month, told reporters as he arrived at the Iranian diplomatic mission.

"We expect to review the existing issues and also exchange views on the ways we can continue our co-operation to resolve all issues."

For the West, the IAEA negotiations are a test of any substantive shift by Iran from what it saw as intransigence under Ahmadinejad.

Chief IAEA inspector Herman Nackaerts used similar language to Najafi: "We are looking forward to discuss how we can intensify the dialogue to resolve all outstanding issues," he said.

John Kerry meets Iran's Mohammad Javad Zarif

U.S. Secretary of State John Kerry, left and Iran's Foreign Minister Mohammad Javad Zarif meet with foreign ministers representing the five permanent member countries of the UN Security Council in New York on Sept. 26. (Brendan McDermid/Reuters)

For several years, the IAEA has been investigating suspicions that Iran may have co-ordinated efforts to process uranium, test explosives and revamp a ballistic missile cone in a way suitable for a nuclear warhead.

Iran says the allegations are baseless, but has pledged, since Rouhani took office in early August, to expand co-operation with the UN agency. Western diplomats have accused Iran of obstructing the IAEA investigation.

2 prongs of negotiation

Separately, U.S. Secretary of State John Kerry met Iranian Foreign Minister Mohammad Javad Zarif in New York on Thursday, with both expressing cautious optimism.

The two sets of talks represent distinct diplomatic tracks but are linked because both centre on suspicions that Iran may be seeking the capability to assemble nuclear bombs behind the facade of a declared civilian atomic energy program.

Iran says its program is a peaceful bid to generate electricity. But its refusal to curb sensitive nuclear work and lack of full openness with IAEA inspectors have drawn tough Western sanctions, hurting its lifeline oil exports.

Rouhani said this week that Iran would never develop nuclear weapons and called for a nuclear deal in three to six months.

The IAEA has held 10 rounds of talks with Iran since early 2012 to try to resume its stalled inquiry into suspected atom bomb research.

With files from the CBC