US Secretary of State John Kerry pressed his Iranian counterpart on Monday to make "critical choices", six days before a deadline to cut a historic deal that would finally dispel fears about Teheran's nuclear drive.

Going into a second day of negotiations in Vienna with Iranian Foreign Minister Mohammad Javad Zarif, Kerry aimed to "gauge Iran's willingness to make the critical choices it needs to make", a senior US official said.

"That's a pretty serious and potentially lengthy conversation. The secretary will take the time necessary to have that discussion," the official said.

Kerry said, "I am glad that we can have some time to be able to catch up and pick up where we left off."

Zarif appeared to be in for the long haul too, telling Iranian media on Sunday there were still "seven difficult and tough days for discussion".

The mooted accord would kill off for good the fears that Iran might develop nuclear weapons under the guise of its civilian program after a decade of rising tensions and threats of war.

Iran denies seeking an atomic bomb and wants the lifting of all Western sanctions, which have crippled its economy.

The five permanent members of the UN Security Council plus Germany have been negotiating almost nonstop for months after sealing an interim accord in November under which Iran froze its uranium enrichment in return for about $7 billion in sanctions relief.

But the talks to nail down a full treaty have met major sticking points, particularly on how much of Iran's nuclear program to dismantle.

Both sides are also under intense domestic pressure.

Zarif will have to come up with a deal that satisfies Iran's hard-line Islamic leaders, while Kerry is under pressure from the US Congress ahead of November midterm elections not to concede too much.

The United States and Iran have not had diplomatic ties for three decades, making the face-to-face talks between the two top diplomats all the more remarkable.

No breakthrough

On Sunday, Kerry, the foreign ministers of France, Germany and Britain, and the deputy foreign minister of China jetted into the Austrian capital seeking to inject some momentum.

Russia sent a lower-ranking official, but Washington dispelled any speculation - sparked by comments by French Foreign Minister Laurent Fabius last week - of divisions.

But the three European ministers left with no apparent breakthrough.

"It is now up to Iran to decide to take the path of cooperation. ... I hope that the days left will be enough to create some reflection in Teheran," Germany's Frank-Walter Steinmeier said.

"The ball is in Iran's court."

Britain's Foreign Secretary William Hague said it was "very important for Iran to be more realistic".

Before leaving Vienna, Hague said there had been no "decisive breakthrough", and a "huge gap" remained on the key issue of uranium enrichment.

The six powers want Iran to reduce dramatically the scope of its enrichment program, while Teheran wants to expand it.

Israel, the Middle East's sole if undeclared nuclear-armed state and which together with Washington has refused to rule out military action, is opposed to any enrichment by Iran.

Israeli Prime Minister Benjamin Netanyahu warned on Sunday that any nuclear deal leaving Teheran with the capability to pursue this activity would be "catastrophic".

Iranian negotiator Abbas Araqchi was on Sunday publicly sticking by Iran's position on enrichment, which he called "clear and rational".