By Hannah Strange

Iran will only sign up to an international deal on its nuclear programme if it is guaranteed the right to continue enriching uranium "from start to finish", the country's chief negotiator at talks in Geneva said on Thursday.

"No deal that does not include the right to uranium enrichment from start to finish will be accepted," Abbas Araghchi said ahead of negotiations in the Swiss capital aimed at ending the decade-old nuclear dispute. Iran could discuss volumes, levels and locations but "the principle of enrichment is not negotiable", he insisted.

As representives of Iran and six world powers - the US, Britain, France, Russia, China and Germany - prepared for the two days of talks, the negotiator downplayed hopes of agreement. He said there were "major differences" between Iran and world powers, adding: "There is a chance of a deal by tomorrow (Friday) but it's a difficult task."

Mr Araghchi told state television that the main obstacle to agreement was a "lack of trust because of what happened at the last round" - referring to November talks when world powers toughened up the terms of a draft deal - insisting that "as long as trust is not restored, we cannot continue constructive negotiations".

In the run-up to the talks, the key players, including Iran, have expressed optimism that an agreement is within reach. William Hague, the Foreign Secretary, said on Wednesday that the remaining differences between the parties were small. "It is the best chance for a long time to make progress on one of the gravest problems in foreign policy," he said.

But Washington and Tehran reverted to a tougher tone on the eve of the talks on Wednesday, as Ayatollah Ali Khamenei, Iran's Supreme Leader, delivered a bellicose speech insisting there would be no retreat on the country's nuclear "rights" and Washington warned it would be "very hard" to produce a deal.

Ayatollah Khamenei also stoked the ire of two of Iran's negotiating partners, saying America "considered itself superior to mankind" and denouncing France for "kneeling" before Israel – a reference to the fact that French calls for a tougher deal, as demanded by Israel, reportedly scuppered agreement at the last round of talks.

The Ayatollah's tirade drew an angry response from Francois Hollande, the French president, who demanded Iran "provide answers and not provocations" over its disputed nuclear programme.

"It is clear that Khamenei's proposal's could not lead to calm and understanding. Iran must provide answers and not provocations," Mr Hollande said.

Mohammad Javad Zarif, the Iranian foreign minister, sounded a more conciliatory note, saying nuclear development was not "about joining a club or threatening others" and insisting the country's programme was for peaceful energy purposes only.

But it was perhaps partly in response to the Ayatollah's bullish rhetoric that a senior Obama adminstration official warned last night that a deal would be difficult.

"We will have to see because it is hard," the official said. "It is very hard... If it was easy to do, it would have been done a long time ago."

Diplomats say a deal has already been outlined that would freeze Iran's nuclear programme in return for some sanctions relief, a "first step" agreement that would establish a six-month diplomatic window for a long-term settlement to be negotiated.

But it faces vehement opposition from Israel, whose prime minister Benjamin Netanyahu on Wednesday flew to Moscow to lobby President Vladimir Putin to tighten up the terms of the deal, as well as conservative US congressmen who have defied President Obama to push for fresh economic sanctions against Iran.