Iran has said that 90% of technical nuclear disagreements have been resolved with the United States and its negotiating partners, and expressed optimism about meeting a late March deadline for a framework deal.
In an unusually upbeat assessment, Iranian's nuclear chief Ali Akhbar Salehi was optimistic about seeing through negotiations.
The comments were among the most promising to date as the talks over the nuclear deal entered their second extension.
"The main issues have been closed," he told Iranian state TV. "I hope that in the remaining time we can close this."
Both sides are reported to be moving closer on limitations on Iran's nuclear activities that could be retooled to make weapons.
In exchange, the West would progressively lift economic and political sanctions.
While US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif have so far taken the lead in the talks, Salehi and US Energy Secretary Ernest Moniz joined the debate last month to try and iron out the technical differences.
In separate comments to reporters in Lausanne, however, the head of Iran's Atomic Energy Organization said common ground on one "final item" still was missing.
Salehi hinted that once this issue resolved, "we can say that on technical issues, things are clear on both sides".
"Of course there are many details, but I can say that, as a whole, I am optimistic" about a deal before deadline, Salehi added.
Speaking to the Associated Press, a senior US official was less bullish. He said the sides had made progress but still had a ways to go in eliminating differences on what Tehran had to do for a gradual end to sanctions.
Most of the disputes focus on technical issues like the numbers of centrifuges which Iran would be allowed to operate as part of an agreement.