With the top US and Iranian diplomats meeting Saturday in Geneva one month before a deadline for a historic nuclear deal, demands for UN inspections of Iranian military bases appear to be becoming a problem.

Tehran is uneasy about letting foreigners go poking around such sites, saying that since no nuclear material is present, the International Atomic Energy Agency (IAEA) watchdog has no right to enter them.

But the six powers negotiating with Iran want the IAEA to be able to visit them in order to investigate claims of any suspicious activity -- past and future -- that could indicate attempts to build a bomb.

"The Western powers cannot accept a deal that precludes IAEA access to military sites," Mark Fitzpatrick, International Institute for Strategic Studies analyst, told AFP, calling it "politically indefensible".

Last week supreme leader Ayatollah Ali Khamenei said Tehran "will not allow any inspections of military sites by foreigners" or the "interrogation" of nuclear scientists by the Vienna-based IAEA.

France's Foreign Minister Laurent Fabius shot back on Wednesday, saying any deal without access to military sites "will not be accepted".

This prompted a rebuke on Thursday by Iran's Foreign Minister Mohammad Javad Zarif, who called on "my negotiating partners to refrain from making excessive demands".

"People need to have their foot in reality, not in illusions," said Zarif, who is due to hold talks with US Secretary of State John Kerry in Geneva on Saturday.

- Framework deal -

On April 2 Iran and the "P5+1" -- the US, China, Russia, France, Britain and Germany -- agreed to the main outlines of a deal that they hope will end the long-running crisis over Iran's nuclear programme.

Diplomats as well as technical and legal experts have been working hard in Vienna and elsewhere since then to turn the outlines into a final accord by June 30.

If it can be finalised, the deal will see Iran dramatically scale down its nuclear activities to make any dash to make a bomb virtually impossible.

Iran maintains that its nuclear activities are only for peaceful purposes.

The deal will see the IAEA keep even closer tabs than it already does on Iran's nuclear sites -- which Iran accepts, according to April's joint statement issued in Lausanne, Switzerland.

In return, painful sanctions will be lifted.

But the IAEA also wants Iran to address indications that before 2003, and possibly since, Iran's nuclear programme had what it calls "possible military dimensions".

A probe into these allegations, rejected by Iran, has been stalled since August, an IAEA report confirmed Friday. One of the sites it wants to inspect is the Parchin military base.

In addition, the powers want the final deal to give the IAEA the right to probe any suspicious activity further down the line.

This may require the IAEA to visit locations not necessarily declared as containing nuclear material, some of them military, and to talk to certain Iranian scientists.

According to one Western diplomat, the issue of inspections is "one of the legs of the stool. It's not the only one, but if it's not there, the stool will collapse."

"Kerry will underline to Zarif the importance of inspections to the six powers," the diplomat said on condition of anonymity.

- Additional Protocol -

The catch for Iran is that it agreed in April to implement the "Additional Protocol" of the Nuclear Non-Proliferation Treaty.

This document, IAEA head Yukiya Amano told AFP this week, gives the nuclear watchdog "the right to request access at all locations, including military ones".

Comments this week in parliament from Abbas Araghchi, Zarif's deputy, make clear that his team in fact accepts this.

But at the same time, with some Iranian hardliners seeing the mooted deal as going too far, Araghchi stressed that the IAEA would have only what he called "managed access".

This means that any visit to a military base would be tightly controlled, with inspectors unable to wander around where they like inside a base, snapping photos as they go.

"The inspection regime that the parties have agreed to does not amount to 'inspectors anywhere, anytime', which no sovereign country would ever accept," International Crisis Group expert Ali Vaez said.

"Iran has also already allowed the IAEA to conduct inspections at military sites at least 12 times. So to preclude that now would be moving backwards," Fitzpatrick said.