The timing of sanctions relief is the main sticking point in nuclear talks that resumed on Wednesday with a meeting between delegates from Iran and the European Union.
Arriving in Vienna, Deputy Foreign Minister Abbas Araqchi reiterated Iran's position: "All the economic sanctions should be lifted on the day that the deal is implemented," Iranian news agency Tasnim reported.
The United States says the sanctions, imposed on Iran by countries concerned that its nuclear program could be aimed at weapons development, would have to be phased out gradually.
Referring to concerns that the U.S. Congress might try to delay lifting sanctions, Araqchi told state television that President Barack Obama's administration was "responsible to ensure that its commitments, particularly sanctions-related ones, are fulfilled."
Iran and the other countries in the talks -- China, France, Russia, Britain, the United States and Germany -- reached a tentative deal on April 2 and now aim to finalize the details by self-imposed end-June deadline.
"We think it is possible to reach a fair and just agreement ... by the end of June or even before that," Araqchi said. Iran says its nuclear program is entirely peaceful and accuses the West of using the issue as an excuse to hurt the Islamic Republic.
Tehran's commitments on technical nuclear issues under any final deal would be monitored by the International Atomic Energy Agency, a body that Iran's envoy to the Vienna-based agency said Tehran trusted to do the job, Iran's Press TV reported.
Talks between Araqchi and EU political director Helga Schmid are due to continue into Wednesday evening and through Thursday. Delegates from the six powers, including U.S. Under Secretary Wendy Sherman, are expected to join the talks on Friday, according to Iran's Fars news agency.
Iran's foreign ministry spokeswoman, Marzieh Afkham, told Iranian television on Wednesday that U.S. Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif "discussed Iran's nuclear issue by phone last night."
Zarif was quoted by Iranian media as saying he would meet Kerry on Monday in New York on the sidelines of a nuclear Non-Proliferation Treaty review conference at the United Nations.
(Additional reporting by Parisa Hafezi in Ankara and Adrian Croft in Brussels; Editing by Louise Ireland and Robin Pomeroy)