Vienna: US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif held talks in Vienna on Wednesday seeking to inject momentum into troubled talks about Tehran`s nuclear programme.
The five permanent members of the UN Security Council plus Germany (the P5+1) want Iran to scale down its nuclear activities in order to make any dash to make a bomb extremely difficult.
In return Tehran, which denies seeking to develop nuclear weapons, wants the lifting of UN and Western sanctions that are causing its economy major problems.
In July after months of intense talks, negotiators gave themselves four more months, until November 24, to strike a deal. Now there is speculation about a fresh extension.
Herewith a look at the main issues:
ENRICHMENT: The thorniest problem by far is uranium enrichment, a process rendering the material suitable for power generation and medical uses with centrifuge machines but also, at high purities, for a bomb.
Ayatollah Ali Khamenei, supreme leader, said in July that Iran wants to ramp up its enrichment capacities to industrial level. But the powers want Iran to slash them. Both sides have called for more "realism" on this point.
PROGRESS: Progress has been made in other areas. These include greater oversight for UN inspectors, on the future of Iran`s existing uranium stockpile and what Kerry called a "different purpose" for Fordo, Iran`s second main enrichment site under a mountain near Qom.
Another is Iran`s apparent willingness to change the design of a new reactor it is building at Arak in order to ensure that it produces much less plutonium, the alternative to highly enriched uranium for a bomb.
TIMING: Apart from enrichment there are other tricky aspects, not least the duration of the mooted accord. Washington wants Iran`s nuclear activities limited for a "double-digit" number of years, Tehran considerably less.
Another difficulty is the pace at which sanctions would be lifted and how to tie the relief to certain "milestones" reached by Iran. The lifting of sanctions by the UN Security Council and a sceptical US Congress also presents legal difficulties.
SKELETONS IN THE CLOSET: Another potential stumbling block is the UN atomic watchdog`s probe into the "possible military dimensions" of Iran`s programme -- alleged work on developing a nuclear weapon before 2003 and possibly since.
After years during which Tehran rejected these allegations out of hand, progress at last began to be made this year but Iran failed to provide information on two out of around 12 areas of suspicion to the International Atomic Energy Agency by an August 25 deadline.
ANOTHER EXTENSION?: Because of the difficulties the two sides are experiencing in seeing eye to eye, experts have begun to speculate about yet another extension.
This could include locking in measures to do with Fordo, for example, or Arak, in a so-called "Interim Plus" agreement.
"It is quite worrying that this late in the game there are still conceptual differences between the parties," analyst Ali Vaez from the International Crisis Group told AFP.
"As such, a full-fledged agreement by November 24 no longer appears likely. What is still possible is a breakthrough that could justify adding more time to the diplomatic clock."
Whether this would fly with hardliners in Washington and Tehran -- and allegedly nuclear-armed Israel, Iran`s arch-foe -- long since sceptical about the whole process is a moot point.
"But policymakers in Washington, Tehran and Israel must remember that the worst-case scenario is a failure of the negotiations. Nobody wins if the talks break down," said Kelsey Davenport from the Arms Control Association.