Iran and six world powers resumed talks meant limit the Islamic Republic's nuclear programme in exchange for sanctions relief, but "details and wording" of a document that could pave the way for a first-stage agreement remained a sticking point.
Senior envoys from both sides gathered in Geneva on Thursday to try to wrestle with a draft they hoped would be acceptable to both Tehran and the five permanent members of the UN Security Council as well as Germnay, known as the P5+1.
Although both sides are still optimistic that a deal resulting in Tehran scaling back its nuclear programme in exchange for sanctions relief could be reached, Iran's supreme leader, Ayatollah Ali Khamenei, has said his country will continue to "exercise its nuclear rights", indicating there will be little room for concessions.
But Iranian officials may have to deal with increased sanctions against the country should the talks fail to bear fruit.
"The Senate must be prepared to move forward with a new bipartisan Iran sanctions bill when the Senate returns after the hanksgiving recess. I'm prepared to do just that," said Harry Reid, majority leader of the United States Senate.
Iran is already reeling from crippling sanctions imposed by the UN, the EU and the US to stop its nuclear programme which the West suspects seeks to develop a nuclear bomb. Tehran insists its programme is intended for civilian use.
Al Jazeera's James Bays, reporting from Geneva, said the media had been kept away from the talks.
"There are two ways of looking at this. You can say, quite accurately, that 'They are very, very close to the deal.' You can also say, 'This is the most difficult part of the negotiations' because they're dealing with those sticking points," he said.
Abbas Araghchi, the Iranian deputy foreign minister, suggested that the momentum of a previous round had slowed, as top EU diplomat, Catherine Ashton, and the Iranian foreign minister, Mohammad Javad Zarif, discussed the draft.
When the two broke for lunch, Zarif said they were discussing "details and wording" of the document but pointed to what his country sees as a potential problem ahead.
"We expect the West to have a united stance over [the] draft," he told Iranian state TV, alluding to what Iran says are hurdles to reaching a first-stage deal.
Araghchi suggested those differences had set back the talks, telling The Associated Press news agency: "What we are trying now is to rebuild confidence that we lost in the previous round of negotiations."
He spoke of some unspecified "misunderstanding or ... mismanagement in the previous round," and of the "difficult job" of trying to bridge differences.
Iran has signed the nuclear non-proliferation treaty, which affords nations the right to civilian nuclear technology in exchange for not acquiring nuclear weapons.
However, the UN's nuclear watchdog, the IAEA, accuses Tehran of failing to disclose its activities and not allowing full inspections.
Previous rounds of talks have all ended in failure, with Iran's continued enrichment of uranium to near a "weapons-useable" threshold proving a major obstacle.
Tehran has also continued work on its Arak heavy water reactor, which would produce plutonium, which can be used in a future nuclear bomb.
The six world powers have demanded Iran suspend enrichment of uranium to 20 percent fissile purity - a significant advance towards the threshold for bomb fuel - as well as limiting its enrichment capacity and postponing the Arak project.
Those countries have been joined by Israel, which has threatened military action against Tehran over its nuclear programme, although the Jewish state is widely suspected of owning a nuclear arsenal and has not signed the Nuclear Non-Proliferation Treaty.