WASHINGTON: Major powers and Iran finessed how UN inspectors will get access to Iranian military sites in Tuesday's nuclear agreement, with a formula that gives the United Nations strong inspection powers while allowing Tehran to save face.

Deep in the deal's details is a procedure under which Iran would have to provide access to suspect sites, including at its military facilities, within 24 days. If Iran refused, it would face the possibility of UN sanctions being slapped back on it.

The procedure was crafted to ensure UN inspectors could get access to allay their suspicions about Iran's nuclear activities. But it does not explicitly force Iran to admit that its military sites could be open to foreign inspections, leaving some uncertainty over the access Iran will allow in practice.

In Tuesday's landmark deal, Iran and six major powers struck a compromise under which Tehran will limit its nuclear programme in return for relief from economic sanctions.

One of the most controversial issues in the negotiations was whether the UN's International Atomic Energy Agency (IAEA) would be able to visit military sites if they had questions about suspected nuclear activities or facilities within them.

The matter became even harder to resolve, diplomats said, after Iran's Supreme Leader, Ayatollah Ali Khamenei, on June 23 said granting access to Iran's military sites was a "red line."

In the end, Britain, China, France, Germany, Russia and the United States agreed on language with Iran that requires more of Tehran than the existing global nonproliferation system while avoiding a direct mention of the sensitive military site issue.

"This is rather clever and reflects the interests of all sides," said George Perkovich, vice president of studies at the Carnegie Endowment for International Peace think tank.

INSPECTIONS, 'WITHOUT SAYING SO'

Under the Nuclear Non-Proliferation Treaty's "Additional Protocol," the IAEA may ask for "managed access" to any site, including military, but a country can legitimately bar access by tying the UN nuclear watchdog up in endless negotiations.

This deal aims to close such loopholes with a process under which Iran would give access or otherwise allay IAEA concerns within 24 days, a time frame experts say is tight enough to keep it from sanitising unauthorized nuclear work.

Iran and the IAEA have 14 days to resolve disagreements among themselves. If they fail to, a joint commission comprised of eight members - the six major powers, Iran and the European Union - would consider the matter for a week.

A majority of the eight could then inform Iran of the steps it would then take within three more days.

Majority-rule means the United States and its European allies -- Britain, France, Germany and the EU -- could insist on access or any other steps and that Iran, Russia or China could not veto them.

"This almost inevitably means inspections but without saying so. That’s why diplomats make the big bucks," Perkovich added.

Nonproliferation experts said the regime falls short of the "anywhere, anytime" inspections demanded by critics of the deal, including many Republicans, but said that would only be possible in a country that has been defeated militarily.

"It’s not a perfect procedure. It would be good to get no notice inspections, but that simply wasn’t in the cards," said Bob Einhorn, a nonproliferation specialist at the Brookings Institution think tank and former US negotiator with Iran.

NO ACCESS TO IRANIAN "BEDROOMS"

In hailing the agreement on Tuesday, US President Barack Obama said it meant that "inspectors will have 24/7 access to Iran’s key nuclear facilities."

Obama, who on Wednesday said the deal represented the "most vigorous inspection and verification regime, by far, that has ever been negotiated," was referring only to Iran's declared nuclear sites.

Sites that the IAEA has suspicions about, including any that may be within Iran's many military complexes, fall under the separate procedure with its 24-day time limit.

The word "military" occurs only once in the agreement, where it says that access requests would not be aimed at interfering with Iranian military or other national security activities.

Senior Iranian officials said they would provide the "managed access" called for under the Additional Protocol and said little about the additional procedures stipulated under the new deal.

"Managed access" is a mechanism to allow the minimum needed IAEA oversight to ensure there is no diversion to clandestine nuclear or nuclear-related activities, while limiting access to protect a legitimate military or industrial secrets.

"We have nothing to hide. We have always cooperated with the IAEA and allowed them to visit our sites," a senior Iranian official told reporters in Vienna on Tuesday. "However, it does not mean that we are going to share our intelligence with others or allow them to enter to our bedrooms to investigate."

US officials said they believed that Tehran had committed to providing access to any site, including military, though they acknowledged the possibility it might refuse.

If it found Iran to be in breach of the deal, the United States could, single-handedly, move to "snap back" UN sanctions on Iran.

Under an agreement among Britain, China, France, Russia and the United States, the UN Security Council's permanent members who each have veto power, a resolution to re-impose sanctions would be drafted in such a way that none could block it.

Reinstating the UN sanctions in full would be to a wield a heavy hammer against Iran and one that the major powers might be loathe to use.

However, a senior US official raised the possibility of re-imposing some but not all sanctions through a "partial snapback," making the punishment more a scalpel than hammer.

Asked if Washington expected Tehran to honour demands for access, and hence avoid any need to reimpose UN sanctions, a senior US official told Reuters: "I hope so."