Tehran insists the programme is entirely devoted to civilian purposes.
World powers and Iran have set an end of March deadline for a framework agreement, with a further four months for the technical aspects to be set.
The talks have missed two previous deadlines. US President Barack Obama has said a further extension would make little sense without a basis for continuing discussions.
Meanwhile in a response to remarks by US officials, Deputy Foreign Minister and Iran's senior nuclear negotiator Abbas Araqchi said Tehran attaches no importance to media hype and continues talks as long as there is respect at the negotiating table, IRNA reports.
"Iranian nation and nuclear negotiators as the sons of the nation will never surrender to threats, bullying, pressures, and force, Araqchi said."
U.S. Secretary of State John Kerry said Saturday that “significant gaps” remained and warned that Washington was ready to walk away from the talks if Tehran didn’t agree to terms demonstrating that it doesn’t want atomic arms.
'We have already faced this kind of American rhetoric and it is unfortunate that they repeat it while it has completely failed,' Araqchi added.
'Both Americans and other members of 5+1 Group (Russia, Britain, France, China, Germany) have experienced several times that political or media pressures do not cause Iran to change its approaches, demands, and stances in the negotiations whatsoever.'