U.S. Secretary of State John Kerry and his Iranian counterpart Muhammad Javad Zarif were meeting at a luxury Geneva hotel for a third session in talks that began Sunday.
The Iranian minister later told reporters the next round of talks will be held on the sidelines of the UN Human Rights Council in Geneva next week.
The talks are the latest in a string of meetings between the two men in a bid to forge a long-elusive nuclear deal.
There is now a heightened sense of urgency as the clock ticks down towards a March 31 deadline to agree on a political framework for the deal.
A senior State Department official said "a full schedule of meetings" was planned Monday with the participation of all the main negotiators and experts, touching on "virtually every topic".
"These meetings are steps in a long and tough process," the official said.
Zarif told Iranian media that mid-level bilateral talks had produced "good discussions but no agreements", and some differences remained.
"The fundamental gap, in my view, is psychological. Some Western countries, the United States in particular, see sanctions as an asset, a lever to exert pressure on Iran. As long as this thinking persists it will be very hard, difficult to reach a settlement."
Iran's Deputy Foreign Minister Abbas Araghchi, who is taking part in the discussions, said "key questions" remained unresolved.
"All parties are negotiating with seriousness and determination, but we haven't found solutions to key questions," he told Iranian national television late Sunday.
"The gap still exists, differences exist."
Before the talks, Kerry also acknowledged that there were "still significant gaps. There is still a distance to travel".
U.S. and Iranian diplomats have been meeting in Geneva since Friday, and senior negotiators from the so-called P5+1 group of Britain, China, France, Russia, the United States and Germany and held parallel negotiations with Tehran on Sunday to help drive the talks forward.
In a sign of the growing push for an accord, U.S. Energy Secretary Ernest Moniz is taking part in the talks for the first time, as is Ali Akbar Salehi, the director of the Iranian Atomic Energy Organization.
Both men led five hours of negotiations on Saturday, before Kerry's arrival.
But Kerry played down the importance of adding new negotiators to the mix, saying Moniz was present because of the "technical" nature of the discussions.
Zarif said the inclusion of Moniz and Salehi reflected a need "for higher level people with all-embracing command over all issues".
The presence of a close aide and the brother of Iranian President Hassan Rouhani, Hussein Fereydoon, meant better "coordination with the president", he added.
The talks took place behind closed doors with no customary photo opportunity for journalists covering the meetings.
Kerry warned Saturday that U.S. President Barack Obama had "no inclination whatsoever to extend these talks beyond the period that has been set out".
Ali Akbar Velayati, the diplomatic advisor to Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei, shot back Sunday that "if American leaders don't want to negotiate, it's up to them, but they were the ones who were after negotiations".
Araqchi said Iran will quit the nuclear talks with the P5+1 should the other parties try to impose their will on the Islamic Republic.
He said the nuclear negotiations should be geared toward meeting the interests of both sides and a win-win solution.
"If that does not happen, and if one side attempts to impose its will in the talks through means other than negotiating, we, too, will certainly be under no compunction to leave the negotiating table,” Araqchi said.
"But”, he said, "that is not the situation we are in right now.”
Araqchi also said that Iranian negotiators will continue the talks with strong will and determination and "whenever we sense that the negotiations are not proceeding in line with the nation and the country’s interests and toward the defined objectives, we will definitely quit negotiating”.
He further said that it has been proven to the United States and the other members of P5+1 that "political and media pressure can by no means cause the Islamic Republic of Iran to alter its method, demands and position in the talks”.
Araqchi said the remarks by Kerry will have no effects on the talks, and that Iran will continue the talks as long as the language of respect is used during the talks.
He also evaluated the atmosphere of the talks as being "constructive and useful”.
Kerry had said Obama believed it was "imperative to be able to come to a fundamental political outline and agreement within the time space that we have left".
But Zarif said Rouhani would not accept a small, short-term agreement, nor a broad accord that left room for interpretation.