"It's an important step forward, but by no means the end of the process," Amano told The Associated Press in Tehran. "There is still much work to be done."
Western leaders, meanwhile, were keen to display a unified front after suggestions that France had broken ranks in Geneva and demanded more concessions from Iran on enrichment levels and an under-construction heavy water reactor that produced a greater amount of plutonium by-product, which could be used in weapons production.
U.S. Secretary of State John Kerry said it was Iran that put the brakes on reaching a first-phase agreement.
"There was unity but Iran couldn't take it," Kerry said during a stop in Abu Dhabi. He added: "The French signed off on it, we signed off on it."
Kerry told the BBC on Monday that negotiators had been "very, very close ... extremely close" to reaching a deal with Iran.
"I think we were separated by four or five different formulations of a particular concept," he said.
In the BBC interview, Kerry acknowledged "the French have been more vocal about one thing or another." But he said, "the fact is that we had a unity on Saturday in a proposal put in front of the Iranians. But because of some the changes they felt they had to go back and change it."
British Foreign Secretary William Hague said the world powers presented a united front to Iran at the weekend talks that failed to reach an accord, and although "some gaps" remained between parties at the talks, "most of those gaps are narrow."
In Jerusalem, Israeli Prime Minister Benjamin Netanyahu has acknowledged that an overall deal is likely between Iran and world powers, which would undercut Israeli threats to launch military action against Iranian nuclear sites. Yet he hailed the delay as a chance to "achieve a much better deal."
"The target date for this deal is the date on which a good deal will be achieved that will deny Iran a military nuclear capability," he told Israel's parliament Monday.
Iran has categorically denied it seeks nuclear arms and insists its only seeks reactors for energy and medical applications. Iranian officials portrayed the expanded U.N. access as further sign it seeks to work with the West.
Under the plans, Iran would allow inspectors a first-time visit of its key Gachin uranium mine on the Gulf coast and give broader access to the heavy water facility being built in the central city of Arak.
Heavy water reactors use a different type of coolant to produce a greater amount of plutonium by-product than conventional reactors. Inspectors from the U.N.'s International Atomic Energy Agency, or IAEA, have already visited the reactor site but seek more extensive examinations.
Plutonium can be used in nuclear weapons production, but separating it from the reactor by-products requires a special technology that Iran does not possess.
The new accord also calls on Iran to provide more details on its nuclear program including proposed new reactor sites and all planned research reactors. This is important because such facilities use 20 percent enriched uranium, which is the highest level acknowledged by Iran and a key aspect of the ongoing nuclear talks. Halting the 20 percent enrichment which is several steps away from weapons-grade is a key goal of Western envoys, for which they may offer Iran a possible easing of U.S.-led sanctions.