US Secretary of State John Kerry has pressed his Iranian counterpart to make “critical choices”, six days before a deadline to cut a historic deal that would finally dispel fears about Tehran’s nuclear drive.

Going into a second day of negotiations on Monday in Vienna with 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.

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

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 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 the atomic bomb and wants the lifting of all UN and Western sanctions, which have crippled its economy.

The five permanent members of the UN Security Council plus Germany have been negotiating almost non-stop 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 hardline Islamic leaders, while Kerry is under pressure from Congress ahead of November mid-term 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.