The comments by Ayatollah Mohammad Emami-Kashani, which state media said were replicated nationwide, are a further boost for President Hassan Rouhani's government, which has been pushing for an accord.
With a June 30 deadline for a final deal, the big hurdle between Iran and the P5+1 group (Britain, China, France, Russia and the United States plus Germany) remains the mechanism for lifting a slew of sanctions imposed on the Islamic republic for pursuing its nuclear programme.
He praised Iran's supreme leader Ayatollah Ali Khamenei for the document that laid out the key parameters of a deal, but cautioned on sanctions.
"They eventually accepted to lift them but what the process of the sanctions relief might look like depends on the future talks," said Emami-Kashani.
"The Western side should know that just as Iran will hold to its commitments in the set framework, they also must keep their promises. If they break their promises, so will Iran."