VIENNA (Reuters) - Iran has stuck to "unworkable and inadequate" positions in nuclear talks with six world powers despite a looming deadline for a deal to end sanctions against Tehran in exchange for curbs on its atomic programme, a U.S. official said on Saturday.
"We are still very far apart on some issues and obviously on enrichment capacity," the senior U.S. administration official told reporters hours before Secretary of State John Kerry was due to arrive in Vienna to join the talks with Iran.
The major powers want Iran to scale back its nuclear programme to deny it any capability to quickly produce atomic bombs. Iran says its activities are entirely peaceful and want crippling sanctions lifted as soon as possible.
"We have made some progress but on some key issues Iran has not moved, from our perspective, from unworkable and inadequate positions that would not in fact assure us that their programme is exclusively peaceful," the U.S. official said.
In view of still wide differences in positions, some diplomats and experts believe the negotiations may need to be extended beyond a self-imposed July 20 deadline for an accord.
However, another senior U.S. official said it was difficult to consider extending the talks between Iran and the United States, France, Russia, China, Britain and Russia without first seeing "significant progress on key issues".
"If (a comprehensive agreement) can't happen by July 20 both the administration and Congress are on the same page, which is that we obviously have to consider all of our options."
"But it would be hard to contemplate things like an extension without seeing significant progress on key issues and that is what we are going to be looking for here over the next few days," the official said, also speaking on condition of anonymity.
The first U.S. official criticised recent comments from senior Iranian officials, including Supreme Leader Ayatollah Ali Khamenei, who said this week Iran would need to significantly increase its uranium enrichment capacity.
"The numbers we have seen ... go far beyond their current programme and we have been clear that in order to get an agreement that their current programme would have to be significantly reduced," the official added.
Khamenei suggested that Iran intended to dramatically increase the number of enrichment centrifuges it maintains from around 10,000 operational machines now to around 190,000.
Western powers want Iran to reduce the number to a few thousand, diplomats have said.
Another sticking point in the talks is the duration of any future limits on Iran's nuclear programme. Tehran would like them to last only around five years, while the Western powers would like around 20 years.
The first senior U.S. official declined to give specifics, but said the United States wanted the limits to last for years, in the double digits.
The official also appeared to respond to comments from France's foreign minister, who said this week that "differences in approach" between Russia and some of the other five world powers negotiating with Iran had appeared in the past few days.
"We remain very united," the official said. "Everybody has their national positions of course."
The French, British and German foreign ministers will join Kerry in the Austrian capital on Saturday. The Russian and Chinese foreign ministers will not attend due to a meeting of the so-called BRICS developing countries - Brazil, Russia, India, China and South Africa. But Moscow and Beijing will send senior diplomats to Vienna for the talks.
The second U.S. official was asked about plans by Kerry and other Western foreign ministers to discuss the latest Israeli-Palestinian conflict in the Gaza Strip.
"Iran has a long-standing record of supplying weapons, rockets to various terrorist groups in Gaza, including Hamas, (and) those rockets are being used to fire at civilian areas," the official said.
"Iran has a responsibility to cease and desist from continuing to supply weapons of war there fuelling this conflict and any opportunity that we get to communicate that message to them we will take."