VIENNA (AFP) - US Secretary of State John Kerry held what he called "very tough" talks with his Iranian counterpart in Vienna on Monday just six days before a deadline to cut a historic nuclear deal.
"We are in the middle of talks about nuclear proliferation and reining in Iran's programme, it is a really tough negotiation I will tell you," Kerry said during a second day of high-stakes discussions in Vienna.
His talks with Iranian Foreign Minister Mohammad Javad Zarif were in order to "gauge Iran's willingness to make the critical choices it needs to make," a senior US official said.
It was unclear how long they would last, although Egyptian state media reported that Kerry would visit the country on Tuesday in an effort to broker a truce in Gaza.
Zarif appeared to be in for the long haul too, telling Iranian media on Sunday there were still "difficult and tough days for discussion" ahead until the July 20 deadline.
The mooted accord would kill off for good fears that Iran might develop nuclear weapons under the guise of its civilian programme after a decade of rising tensions and threats of war.
Iran denies seeking the atomic bomb and wants the lifting of crippling UN and Western sanctions.
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 US$7 billion (S$8.6 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 programme 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.
Kerry, along with the foreign ministers of France, Germany and Britain and the deputy foreign minister of China jetted into the Austrian capital on Sunday seeking to inject some momentum.
Russia sent a lower-ranking official, but Washington dispelled any speculation - stoked by comments by French Foreign Minister Laurent Fabius last week - of divisions between the six powers.
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 Tehran," 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.
This activity can produce fuel for the country's sole nuclear plant or, if further enriched, the material for an atomic bomb.
The six powers want Iran to reduce dramatically the scope of its enrichment programme, while Tehran 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 Sunday that any deal leaving Iran with the capability to pursue enrichment activity would be "catastrophic".
Iranian negotiator Abbas Araqchi was Sunday publicly sticking by Iran's position on enrichment which he called "clear and rational".
"As the supreme guide said, the enrichment programme has been planned with the real needs of the country in mind, meaning our need to ensure reactor fuel." If no agreement is reached by next Sunday when the six-month interim accord runs out, all sides can agree to extend the talks for a further six months.
Hague said Sunday that such a move "will only be discussed if no progress can be made. It is still too early."