Negotiators from Iran and six world powers sought Tuesday to overcome major differences and hammer out the text of a momentous nuclear deal just five weeks before a deadline.
This hugely complex accord would see the Islamic republic scale down its nuclear programme to ease concerns that it wants atomic weapons -- something Iran has long denied.
In return Iran is demanding the lifting of all UN and Western sanctions that are hitting its vital oil exports, clogging up its financial system and causing major economic problems.
Such a deal is aimed at resolving one of the trickiest geopolitical problems of the 21st century after a decade of rising tensions, threats of war -- and of nuclear expansion by Tehran.
"The negotiations have already intensified, as we said that they would, and they will continue to do so in the days and weeks leading up to July 20," a senior US official said Monday on the first day of talks in Vienna.
She added however that there are "still significant gaps... and we don't have illusions about how hard it will be to close those gaps, though we do see ways to do so".
"Everyone is aware that we are entering a phase of negotiations where things are getting quite intense," said Michael Mann, spokesman for the six powers' chief negotiator, EU foreign policy chief Catherine Ashton.
- Thorny issues -
The many problem areas include the duration of any final accord, the pace of sanctions relief, Iran's partially-built Arak nuclear reactor and allegations of past efforts to build a bomb.
But the main sticking point remains, as it has been for years, uranium enrichment: a process that can produce nuclear fuel but also, when highly purified, the core of an atomic bomb.
The West hotly disputes Iran's claim that it needs this material for civilian nuclear facilities, saying it only has one power plant -- fuelled by Russia -- and that others are years from completion.
French Foreign Minister Laurent Fabius said last week that Iran should slash the number of centrifuges, machines that enrich by spinning uranium gas at supersonic speeds, to "several hundred".
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But Iran, which has 20,000 centrifuges, 10,000 of them spinning, is believed to want to massively increase its capacities. It is also developing newer, faster machines.
"We are not even in the same ballpark," said Fabius.
- Tick tock -
The parties have set themselves a deadline of July 20, when an interim deal struck in November expires, and many experts believe an extension is already being talked about.
The senior US official denied any such discussion, however, indicating some progress in bilateral US-Iranian talks in Geneva last Monday and Tuesday.
"We not only understood each other better after those two days, but I think we both can see places where we might be able to close those gaps," she said.
On Monday the United States and Iran briefly diverted from the nuclear talks to discuss the crisis raging in Iraq, US officials said.
Both countries, bitter foes since Shiite Iran's 1979 Islamic revolution, are both unsettled by a major insurgency by Sunni militants overrunning large swathes of Iraq in the past week.
Washington was deploying 275 military personnel to protect its Baghdad embassy and was also mulling air strikes, as the UN envoy in Iraq warned of the country's breakup.
Trita Parsi, president of the National Iranian American Council, said the crisis "adds urgency" to securing a nuclear deal with Iran because it would make it easier to "explore regional areas of mutual interest".
But whether any sharing of interests or even US-Iranian cooperation in Iraq could actually help get a nuclear agreement remains to be seen.
"I expect tactical cooperation over Iraq to have as much impact on the nuclear talks as the tactical disagreement over Syria," Ali Vaez of the International Crisis Group told AFP.
"Which means not much."