U.S. President Barack Obama lauded a landmark nuclear agreement with Iran as vindication of his diplomatic approach and a chance for a "new direction" in decades of vexed relations with Tehran.

Obama said the deal -- which would curb Iran's nuclear program in return for substantial international sanctions relief -- cut off "every pathway" to an Iranian atomic weapon.

When U.S. President Barack Obama accepted the Nobel Peace Prize early in his first term, he acknowledged the controversy surrounding his win by saying he was only at the beginning of his "labors on the world stage."

Nearly six years later, having secured a landmark nuclear deal with Iran, he has achieved a major diplomatic victory that staves off the threat of a military conflict with unforeseeable consequences in an unstable region.

Major powers clinched a historic deal Tuesday aimed at ensuring Iran does not obtain the nuclear bomb, opening up Tehran's stricken economy and potentially ending decades of bad blood with the West.

Reached on day 18 of marathon talks in Vienna, the accord is aimed at resolving a 13-year standoff over Iran's nuclear ambitions after repeated diplomatic failures and threats of military action.

Iran and major powers headed into another late night of talks Monday on a deal to rein in Tehran's nuclear ambitions, as negotiators struggled to overcome the remaining few obstacles.

The White House said the marathon discussions in Vienna had "made genuine progress" but suggested they would stretch beyond a Monday night deadline.

The mother of Washington Post journalist Jason Rezaian, on trial in Iran for espionage, told reporters outside a Tehran court Monday that she hopes he will be freed on bail.

Rezaian, the newspaper's Tehran Correspondent, was detained in the capital almost one year ago in a case that has paralleled with Iran's nuclear talks with world powers led by the United States.

A top U.S. Republican said a potential historic nuclear deal with Iran would be a "hard sell" in Congress, as talks neared their final hour in Vienna on Sunday.

"It appears as if the administration's approach to this was to reach whatever agreement the Iranians are willing to enter into," U.S. Senate Majority Leader Mitch McConnell said in an interview broadcast on "Fox News Sunday."

Iranian President Hassan Rouhani said on Sunday the conclusion of a nuclear deal with the major powers was "very close" after more than two weeks of intensive talks in Vienna.

"We have come a long way. We need to reach a peak and we're very close," he said at an iftar meal breaking the Ramadan dawn-to-dusk fast in Tehran.

Negotiators at the Iran nuclear talks are expected to reach a provisional agreement Sunday on a historic deal that would curb the country's atomic program in return for sanctions relief, diplomats said.

Two diplomats at the talks told The Associated Press the envisioned accord will be sent to capitals for review and, barring last-minute objections, be announced on Monday.

Iran has changed a law to make divorce by mutual consent invalid unless couples have first undergone state-run counselling, the country's latest move to tackle a rise in broken marriages.

The measures, reported by media at the weekend, are contained in a new family law that a top official said would be implemented by Iran's judiciary.

Iran should boost trade with its Gulf Arab neighbors if a deal on its nuclear program sees sanctions and an oil embargo lifted, but higher Iranian crude production could worsen tensions within OPEC, analysts say.

As of Saturday, the prospects of such a deal were still very much in question, with no sign of an end to a nail-biting deadlock after 15 straight days of negotiations in Vienna between Iran and major world powers.