A deal would limit Tehran's ability to make an atomic weapon while easing economic sanctions
US Republican leaders have threatened to thwart the deal
Officials face a June deadline for a comprehensive agreement
A senior Iranian official has suggested that Iran is nearing a preliminary deal with the US and five world powers that would lead to a breakthrough eluding them for more than a decade.
The formal pact would limit Iran's ability to make an atomic weapon while easing punitive economic sanctions on the country.
The US was less optimistic, with officials saying the sides had made progress but still had a long way to go in eliminating differences over what Iran had to do for a gradual end to sanctions.
"There's no doubt they have made substantial progress over the past year," Josh Earnest, White House spokesman, said. "Reaching an agreement is at best 50-50."
The sides face two deadlines: an end-of-March date for a preliminary deal, and a June deadline for a comprehensive agreement that fills in the blanks.
A comprehensive agreement that the US says would stretch the time Iran would need to make a bomb from a few months to a year has been a top foreign-policy objective of the Obama administration.
Even a deal by deadline would not end the Iran nuclear controversy, however.
Republican politicians have threatened to scuttle the deal, claiming it is ineffective, and it is expected to further cloud relations between the US and Israel if Benjamin Netanyahu, the incumbent prime minister, forms the next government, either through elections on Tuesday or as the head of a new ruling coalition.
Officials for both sides have said that the talks are making headway on limiting Iranian nuclear activities that could be retooled to make weapons.
In exchange, the West would progressively lift economic and political sanctions.
Still, the comments by Ali Akhbar Salehi, the Iranian nuclear chief, were among the most promising to date.
Only one "final item" remained contentious, he said in Lausanne, Switzerland, where the talks are being held.
He did not specify the "final item".
If that is resolved, "we can say that on technical issues, things are clear on both sides," he said, adding: "As a whole, I am optimistic."
John Kerry, US secretary of state, and Mohammad Javad Zarif, Iranian foreign minister, have taken the lead in what formally remain talks between Iran on one side and the US, Russia, China, Britain, France and Germany on the other.
Focus on technical issues
Most of the disputes focus on technical issues like the numbers of centrifuges Iran would be allowed to operate as part of an agreement.
The machines can enrich uranium up to levels used for the fissile core of nuclear arms, but Iran says it only has energy, medical and scientific aims.
Salehi and Ernest Moniz, the US energy secretary, joined the talks last month to try and iron out the technical differences.
But even if the final-stretch talks turn into a deal, outside interference could hurt chances of its implementation.
A letter last week by Republican senators to the Iranian leadership warning that Congress could scuttle any deal continues to cast a shadow over the negotiating table.
Another senior American official said the issue came up at Monday's Kerry-Zarif meeting as well as a Sunday gathering among senior US and Iranian negotiators.
Both American officials quoted by the Associated Press demanded anonymity because they were not authorised to discuss the talks on record.
The deal taking shape would limit Iran's uranium enrichment and other nuclear activity for at least a decade, with the restrictions slowly lifted over several years.