Key players in talks on Iran's nuclear program warned on Sunday that big gaps remain between Iran and world powers, with only a week left to strike a deal, the BBC reports.
Arriving in Vienna for the talks, U.S. Secretary of State John Kerry said "significant differences" remain - a view echoed by France and the UK.
"We haven't made the decisive breakthrough," the BBC quoted UK Foreign Secretary, William Hague, as having said. "There are very significant gaps."
The talks in Vienna bring together Iran and the P5+1 group, comprising the US, UK, France, China, Russia and Germany.
All the countries except Russia and China have sent their highest-ranking foreign envoys.
"Obviously we have some very significant gaps still, so we need to see if we can make some progress,'' Kerry told reporters before meeting Catherine Ashton, the EU's foreign policy chief.
"It is vital to make certain that Iran is not going to develop nuclear weapons, that their program is peaceful," he added. "That's what we are here trying to achieve.''
Iran and the West have been holding talks in an attempt to turn an interim nuclear agreement reached into November into a permanent deal.
Western diplomats said last week that Iran has reduced demands for the size of its future nuclear enrichment program, even as they added it would not be easy to clinch a deal by the July 20 deadline.
World powers suspect Iran is seeking atomic weapons. Iran strongly denies the charge, insisting that it is enriching uranium to fuel its power plants, and for medical needs.
A deal could see the lifting of oil and trade sanctions on Iran, which seems to have toughened its position. Last Tuesday, Iranian Supreme Leader, Ayatollah Ali Khamenei, said Iran "needs" 19 times more nuclear centrifuges than the amount being offered by world powers.
Iranian nuclear agency head Ali Akbar Salehi echoed Khamenei’s remarks a day later, saying Iran wants to greatly expand its uranium enrichment program despite Western fears that it could be used to make atomic arms.
Meanwhile, Prime Minister Binyamin Netanyahu on Sunday once again warned against the deal being worked out with Iran, saying that any deal leaving Iran with the capability to enrich uranium would be "catastrophic."
"It would be a disaster for the United States and for everyone else," Netanyahu told Fox News.
He added that "a bad deal is actually worse than no deal," defining that as one in which Iran would keep enriched nuclear material and the capability to further enrich uranium in return for monitoring by international inspectors.
"I certainly hope that doesn't happen. I think it would be a catastrophic development, because you know the Middle East is in turmoil, everything is topsy-turvy, the worst militants, Shiites and Sunni radicals are vying with each other who will be the king of the Islamist hill," he said.
"If any one of these sides get their hands on nuclear weapons, all bets are off."