VIENNA (AP) -- Setting up a potential showdown with Iran, the head of the U.N. atomic agency said Tuesday that a nuclear agreement being worked on by Tehran and six world powers would give his experts the right to push for access to Iranian military sites.

International Atomic Energy Agency head Yukiya Amano spoke as negotiators opened a new round of talks aimed at reaching a deal by the end of June.

Iran tentatively agreed last month to open its atomic activities to greater scrutiny as part of the deal, which would require it to commit to curbing nuclear activities that could be used to make weapons in exchange for relief from international sanctions.

But Supreme Leader Ayatollah Ali Khamenei has since set conditions, declaring military sites off limits "to foreigners ... under the pretext of inspections." Brig. Gen. Hossein Salami of the Revolutionary Guards warns that anyone setting foot into an Iranian military facility will be met with "hot lead" - meaning bullets.

In an Associated Press interview Tuesday, Amano said Iran specifically agreed to implement what's known as the agency's "Additional Protocol" when it agreed to the outlines of the deal now being worked on.

The protocol would allow the Vienna-based agency's inspectors much more access than they have now to follow up on suspicions of undeclared Iranian nuclear activities or equipment.

The United States and Russia are among the 147 countries with additional protocols, although the U.S. and some others have provisions designed to ensure that inspections do not compromise national security, something Iran also demands. Still, Amano said the same rules will apply to Iran as to the others that have signed on.

"In many other countries from time to time we request access to military sites when we have the reason to, so why not Iran?" he said. "If we have a reason to request access, we will do so, and in principle Iran has to accept it."

Amano said the agency can request access, clarification or a "short-notice inspection" anytime "there is any inconsistency (or) abnormality" to what Iran has declared as its nuclear work or assets.

In arguing that military facilities must be accessible to the agency, the United States and others at the negotiating table with Iran point to suspicions that Tehran used them in the past to work on nuclear weapons, something Iran denies.

Confirmed nuclear-related work at such facilities includes the manufacture of components of centrifuges, machines that can enrich uranium for uses ranging from generating energy to the cores of nuclear weapons.

Tehran denies any interest in nuclear arms, but with the proposed deal placing strict limits on centrifuge numbers and types, Washington and its allies want to make sure the agency has a complete picture of such activities. Iran says opening up such sites could lead to spying on its military assets.

Iran has backed down on other issues it previously said were not negotiable and it's unclear whether its unequivocal rejection of access to military facilities is also a negotiating stance that will be open to compromise.

If not, though, it could lead to a crisis if the agency tries for access once a deal is struck and Iran turns it down. David Albright of the Institute for Science and International Security says Iranian refusal could even be a deal breaker.

It is also unclear whether the Additional Protocol will aid the agency in kick-starting nearly a decade of stalled investigations of Iran's alleged nuclear weapons work.

While Iran wants quick sanctions relief, diplomats involved in the negotiations say it cannot hope to be completely sanctions-free until the agency delivers a ruling on the alleged work. But Iran has refused to allow agency inspectors access to documents, officials and sites they are interested in.

Asked if the Additional Protocol will help in the agency's probe, Amano said "we don't know yet."

"We are not in position to provide an assessment now," he said. Success "depends very much on the pace and level of cooperation from Iran."

Iranian Deputy Foreign Minister Abbas Araghchi is already at the talks and senior government officials from the United States, Russia, China, Britain, France and Germany are to join in later in the week.

Araghchi told Iranian state TV that a "noticeable segment" of the agreement has been drafted but "there is still disagreement over part of it."

By The Associated Press