VIENNA — A once-promising UN attempt to probe suspicions that Tehran worked on atomic arms is faltering — and with it, hopes that Iran and six world powers can meet their July target date for an overarching nuclear deal.

With efforts to draft the text of an agreement starting in Vienna on Wednesday, both sides say that meeting the informal July 20 deadline remains possible. The US administration gives it a 50-50 chance, and Iranian Foreign minister Mohammad Javad Zarif recently said the talks are progressing at an “unexpectedly fast pace.”

The United States, Russia, China, Britain, France and Germany want to reduce Iran’s present nuclear weapons-making potential. Tehran has been engaging with them over the past six months in exchange for full sanctions relief, even though it insists it has no interest in such arms.

But the UN’s International Atomic Energy Agency is no nearer to closing the books on persistent allegations that Iran worked on nuclear arms in the past. While the IAEA’s probe is formally separate from the talks, the US and its allies insist that Tehran must provide satisfactory explanations to the UN agency as part of any overall deal.

Back in November, Tehran agreed to go into deeper explanations of its work on detonators that have a variety of uses, including sparking a nuclear explosion.

That has not happened. Three diplomats told The Associated Press Monday that in a recent formal response, Iran continues to insist that there is no nuclear link to the detonators. Tehran says they were developed only to set off conventional military blasts, and later for civilian uses.

The IAEA outlined its suspicions in a 2011 report on a wide range of suspected weapons experiments. It said then that Iran’s work on the detonators is of concern, “given their possible application in a nuclear explosive device, and the fact that there are limited civilian and conventional military applications for such technology.”

Signed soon after Iran’s reformist government took office, the November Iran-IAEA deal as seen as important for testing Tehran’s professed willingness to de-escalate tensions over its nuclear program.

The UN agency and its Western members had hoped the agreement would finally mean Iran would crack open the door on what they say was secret nuclear weapons work.

But Tehran’s latest response suggests it is not ready to change its stance.

The IAEA first approached Iran about the detonators six years ago. When told of their latest response, Olli Heinonen, who headed the agency’s Iran investigation until 2010, said it was “pretty much how they explained it in 2008.”

With the clock ticking down on the informal July comprehensive deal target, IAEA chief Yukiya Amano was pushing ahead on the probe nonetheless.

Two of the diplomats said senior agency officials met Monday with Iranian representatives in attempts to persuade Tehran to engage on three additional areas of suspected weapons work even as they sought more answers on the detonators.

The diplomats are involved with international efforts to track and curb Iran’s nuclear program. They did not detail what those new areas could be and demanded anonymity because they were not authorized to divulge confidential information.

Calls to the Iranian mission to the IAEA went to voicemail, while IAEA spokeswoman Gill Tudor said the agency had no comment.

Gary Samore, who negotiated with Iran until last year as US President Barack Obama’s chief adviser on weapons of mass destruction, said he expects the talks to go past July as Iran and the IAEA wrestle over the nuclear arms allegations.

Ultimately, he said Iran may find some “useful fiction” — for instance, claiming that renegade scientists were behind the program or that research was done to understand the nuclear weapons technology possessed by Tehran’s enemies.

“I think that if all the other issues are resolved this issue will be resolved,” he said. “But it will be the last one to be resolved.”

Experts say other, much trickier questions remain to be resolved, though.

Other claims outlined in a major November 2011 IAEA report include alleged explosives testing at the Parchin military base near Tehran that the watchdog says would be “strong indicators” of missile development.

Iran and the six world powers aim to turn into a lasting accord a temporary deal from November under which Iran scaled back certain nuclear activities for six months in return for minor relief from painful UN and Western sanctions.

As part of this sought-after comprehensive accord, as well as reducing in scope its nuclear activities, the six powers want Iran to answer all the IAEA’s outstanding questions.

Iranian Foreign Minister Mohammad Javad Zarif is due to hold a working dinner with the powers’ main negotiator, EU foreign policy chief Catherine Ashton, on Tuesday before the talks start in earnest on Wednesday.